Why Would One Foot Swell and Not the Other?

One-sided foot swelling almost always points to a local problem in the affected limb rather than a body-wide condition. When both feet swell together, doctors think about the heart, kidneys, or liver. When only one foot puffs up, the cause is usually something happening in or near that leg: a blocked vein, damaged lymph vessel, infection, injury, or structural abnormality in the pelvis or leg itself. The list of possibilities is long, and some are urgent while others are slow-building and chronic.

The Asymmetry Rule

A practical principle in medicine holds that symmetrical lower-limb swelling suggests a systemic or central cause of fluid buildup, such as heart failure, while chronic swelling that is asymmetrical usually arises from venous or lymphatic disease in the affected leg.¹ Fluid accumulates in tissue when something pushes more fluid out of blood vessels than the body can drain away. That can happen because venous pressure rises in one leg, because lymph channels are blocked on one side, or because local inflammation makes capillary walls leakier in a specific area. Any of those mechanisms can produce swelling limited to one foot or one lower leg.

Blood Clots in the Deep Veins

A deep vein thrombosis, or DVT, is the diagnosis doctors worry about most when a single leg or foot swells up suddenly. A clot lodged in one of the deep veins of the leg obstructs blood from flowing back toward the heart, and pressure builds behind the blockage. The swelling is typically accompanied by pain, warmth, and sometimes visible redness. Three factors promote clot formation: damage to the vessel wall, sluggish blood flow, and a tendency for blood to clot too easily.² Prolonged immobility (long flights, bed rest after surgery), recent trauma to the leg, cancer, pregnancy, and estrogen-containing medications all raise the risk.

DVT matters not just because of the swelling but because a clot can break loose and travel to the lungs, causing a pulmonary embolism. That is why sudden one-sided leg or foot swelling warrants prompt medical evaluation rather than a wait-and-see approach.

How Doctors Rule DVT In or Out

When you show up with a swollen leg, the clinical workup usually starts with a risk-scoring system. The Wells score combines clinical features like calf swelling, recent immobilization, active cancer, and tenderness along the deep veins into a probability estimate. In one large study, patients who scored low on the Wells criteria and had a negative D-dimer blood test had a less than one percent chance of having a DVT.³ That combination effectively rules out the diagnosis without imaging. In trauma patients specifically, the Wells score has proven reliable for identifying who does and does not need further testing.⁴

When the clinical suspicion is moderate or high, duplex ultrasound of the leg veins is the standard next step. It is noninvasive, widely available, and good at spotting clots. In patients with both a positive Wells score and an elevated D-dimer, DVT was confirmed by ultrasound about four out of five times in one large dataset.³ If DVT is ruled out, the investigation shifts to the other causes described below.

May-Thurner Syndrome and Pelvic Compression

Some people develop one-sided leg swelling not because of a wandering blood clot but because of an anatomical quirk they were born with. In May-Thurner syndrome, the right iliac artery presses on the left iliac vein where the two cross paths in the pelvis. This compression narrows the vein and slows blood return from the left leg, predisposing that leg to swelling and DVT.⁵ The condition can also cause varicose veins, chronic venous ulcers, and, in severe cases, pulmonary embolism.⁶

May-Thurner syndrome is easily overlooked because it produces symptoms that mimic a standard DVT or simple chronic swelling. It tends to affect the left leg specifically and is more commonly diagnosed in younger women. In one reported case, an 82-year-old woman presented with persistent left-leg swelling that turned out to be iliac vein compression by both the right iliac artery and the lumbar spine.⁷ Pelvic tumors, enlarged lymph nodes, and pregnancy can produce similar one-sided venous compression, which is why imaging of the pelvis is sometimes necessary when the cause of unilateral swelling is not obvious.

Chronic Venous Insufficiency

When the small one-way valves inside leg veins stop working properly, blood pools in the lower leg instead of being pushed efficiently back up toward the heart. This condition, chronic venous insufficiency, develops on a spectrum. Early on, you might notice mild ankle swelling that resolves overnight. Over months or years, the swelling becomes persistent, skin changes develop, and ulcers may form around the ankle.⁸ Though venous insufficiency sometimes affects both legs, it frequently starts or predominates on one side, especially if a prior DVT damaged the valves in one leg.

This is one of the most common causes of chronic one-sided foot and ankle swelling in older adults. Risk factors include a history of blood clots, obesity, prolonged standing, and prior leg injuries. Compression stockings, leg elevation, and exercise that activates the calf muscles are the mainstays of management.

Lymphedema

Your lymphatic system is a network of tiny vessels that drain excess fluid from tissues back into the bloodstream. When lymph channels in one leg are damaged or blocked, fluid accumulates and the foot and leg swell. Lymphedema tends to produce a firm, non-pitting type of swelling that worsens gradually and does not resolve with simple elevation the way venous swelling often does.

Lymphedema can be primary (present from birth due to underdevelopment of lymph vessels) or secondary (caused by surgery, radiation, infection, or trauma that damages the lymphatic system). One useful bedside clue is the Stemmer sign: if you cannot pinch and lift a fold of skin on top of the second toe, the test is positive and lymphedema is likely. In a study of 110 patients, a positive Stemmer sign correctly identified abnormal lymphatic function about 92 percent of the time.⁹ However, the test is less reliable at ruling lymphedema out; patients with early-stage disease or a normal body weight sometimes test negative despite having lymphatic dysfunction.

An interesting finding from lymphatic imaging studies is that unilateral swelling does not always mean unilateral disease. Among patients who appeared to have lymphedema in only one leg, about a third actually had abnormal lymphatic drainage in both legs when assessed by scintigraphy.¹⁰ The “normal” leg simply had not yet progressed to visible swelling. This has implications for long-term monitoring and prevention.

Cellulitis and Other Infections

An infection in the skin and soft tissue of one foot can produce dramatic swelling in a matter of hours. Cellulitis, the most common culprit, typically starts after a break in the skin allows bacteria in. The affected area becomes red, warm, swollen, and tender.¹¹ Because the entry point is usually a specific wound, crack, insect bite, or fungal infection between the toes, cellulitis almost always affects just one foot or leg.

Cellulitis can involve the foot alone or spread to the lower leg and thigh. In a series of patients with confirmed cellulitis and leg swelling, the infection was isolated to the foot in roughly a quarter of cases, with the remainder involving the thigh or lower leg.¹² People with diabetes, chronic swelling from other causes, and compromised immune systems are at higher risk. Recurrent cellulitis can also damage lymph vessels over time, creating a cycle where each infection worsens lymphedema, and worsened lymphedema invites the next infection.

Gout and Inflammatory Joint Disease

A gout flare is one of the more dramatic causes of sudden swelling in a single foot. Uric acid crystals tend to deposit in the joint at the base of the big toe, and several features of the foot make it a prime target: it is the coolest part of the body, it has a slightly lower pH than core tissues, and it endures repeated physical impact with every step. All of these conditions reduce uric acid’s ability to stay dissolved in fluid and encourage crystal formation.¹³ A gout attack typically causes intense pain, redness, and swelling over the affected joint, often coming on overnight. The swelling is localized to the joint area and surrounding foot, not the entire leg, which helps distinguish it from a DVT.

Other forms of inflammatory arthritis, including psoriatic arthritis and reactive arthritis, can also cause asymmetric swelling in one foot. These conditions often produce “sausage toes” where an entire toe swells due to inflammation in both the joint and the surrounding tendon sheath.

Charcot Foot in Diabetes

People with longstanding diabetes who have lost sensation in their feet face a unique risk. Charcot neuroarthropathy occurs when repeated unnoticed injuries cause bones in the foot to fracture, dislocate, and eventually collapse. The early stage looks like a swollen, warm, red foot, and because the person often feels little or no pain, the condition is frequently mistaken for cellulitis or gout.¹⁴ If missed, the foot can deform permanently. The condition almost always affects one foot, making it a classic cause of unexplained unilateral foot swelling in someone with diabetic neuropathy.

Baker’s Cyst Rupture

A Baker’s cyst is a fluid-filled sac behind the knee, commonly associated with arthritis or meniscal tears. When one of these cysts ruptures, fluid leaks down into the calf and foot, causing sudden swelling and pain that closely mimics a DVT. In one published case, a woman developed severe swelling in her left calf and foot over six weeks; DVT was the initial suspicion, but ultrasound and MRI ultimately revealed a ruptured Baker’s cyst instead.¹⁵ Complications of Baker’s cysts beyond rupture include nerve compression and, rarely, compartment syndrome.¹⁶ The key clinical point is that this diagnosis often surfaces only after DVT has been excluded by imaging.

Complex Regional Pain Syndrome

After an injury, surgery, or even a minor fracture in a limb, a small number of people develop a disproportionate pain response accompanied by swelling, color changes, temperature differences, and sweating abnormalities in the affected limb. This condition, complex regional pain syndrome (CRPS), arises from dysfunction in pain signaling and the autonomic nervous system. Edema is one of its hallmark features.¹⁷ The swelling in CRPS tends to be persistent, may fluctuate with activity, and is usually confined to the affected limb. If you had an injury or surgery on one foot and the swelling, pain, and sensitivity seem out of proportion to what the original injury should have caused, CRPS is worth discussing with your doctor.

Stroke and Nerve Damage

After a stroke, the paralyzed or weakened leg frequently swells. The calf muscles normally act as a pump, squeezing veins with every step to push blood back up toward the heart. When one side of the body is weakened or immobile, that pump fails. A study of hemiplegic stroke patients found that the affected lower leg consistently developed more edema than the unaffected side, and neuromuscular electrical stimulation aimed at reactivating the calf pump was among the approaches that reduced swelling.¹⁸ The same principle applies to any condition that immobilizes one leg, including nerve injuries and spinal cord problems affecting just one side.

Vascular Malformations

Arteriovenous malformations are abnormal tangles of blood vessels where arteries connect directly to veins, skipping the normal capillary network in between. When these occur in a leg or foot, high-pressure arterial blood flows directly into thin-walled veins, causing venous hypertension. Over time, this leads to swelling, pain, skin changes, and sometimes ulcers.¹⁹ Vascular malformations are present from birth but may not cause noticeable swelling until adolescence or adulthood when they enlarge. They are uncommon but important to consider when unexplained unilateral swelling persists and standard workups are unrevealing.

Insect Bites and Localized Reactions

A single insect bite on the foot can trigger enough inflammation to cause visible swelling, and in rare cases the aftermath is severe. In one documented case, a young woman developed progressive painful swelling after an insect bite on the left calf, which escalated to involve the knee and foot with blistering over three days. Laboratory testing showed markedly elevated white blood cell counts, and the case ultimately progressed to compartment syndrome requiring emergency surgery.²⁰ Most insect bites produce only mild, short-lived swelling, but scratching the site can introduce bacteria and set off cellulitis. If swelling from a bite keeps expanding, turns red and hot, or is accompanied by fever, treat it as an urgent problem.

Filariasis in Tropical Regions

Globally, one of the most common causes of severe one-sided leg and foot swelling is lymphatic filariasis, a parasitic infection transmitted by mosquitoes. The parasitic worms lodge in the lymphatic system, damage the vessels, and progressively block lymph drainage. In endemic countries across tropical Africa, South and Southeast Asia, and the Pacific, filariasis is the leading cause of lymphedema.²¹ The infection is caused by specific roundworm species, with one being by far the most widespread.²² In advanced cases, the limb can become massively enlarged, a condition historically called elephantiasis. This is not a concern for people who have never lived in or traveled to endemic areas, but for the hundreds of millions at risk worldwide, it is the single most important cause of the symptom this article describes.

When to Seek Care and What to Expect

Not every instance of one-sided foot swelling is an emergency, but certain patterns demand prompt evaluation. Sudden onset with calf pain and warmth suggests a possible DVT and warrants same-day assessment. A foot that is red, hot, and spreading in redness may be cellulitis and needs antibiotics sooner rather than later. Swelling after a known injury that seems to be worsening rather than improving, or swelling accompanied by numbness and loss of pulse, raises concern for compartment syndrome, which is a surgical emergency.

For more gradual or persistent swelling, the diagnostic process usually begins with a physical exam and blood work, including a D-dimer test if DVT is in the differential. Duplex ultrasound of the leg veins is the standard first imaging study. If the ultrasound is normal and the swelling persists, further investigation may include CT or MRI of the pelvis to look for compression of the iliac vein, lymphoscintigraphy to evaluate lymphatic drainage, or blood tests for uric acid if gout is suspected. The pattern, location, and timing of the swelling, combined with your medical history, will guide your doctor toward the right workup. The important thing is not to dismiss single-foot swelling as trivial: the asymmetry itself is a clue that something specific and identifiable is going on.