Swelling in just one leg points to a local problem in that limb, not a body-wide condition like heart or kidney disease (which typically causes both legs to swell). The most common causes are a blood clot, an infection, a problem with the veins or lymphatic system, or an injury. Some of these are urgent, so understanding the differences matters.
Blood Clots in the Deep Veins
A deep vein thrombosis (DVT) is the first thing clinicians rule out when one leg swells. A clot blocks blood from draining out of the leg efficiently, causing fluid to build up in the tissues. The classic signs are leg pain or cramping that often starts in the calf, a feeling of warmth in that leg, and a change in skin color to red or purple.
Certain situations raise your risk significantly: being immobilized for three or more days, having surgery in the past four weeks, a history of prior clots, active cancer, or estrogen use (such as birth control pills or hormone therapy). A DVT on its own is serious, but the real danger is that part of the clot can break free and travel to the lungs, creating a pulmonary embolism. If you develop sudden shortness of breath, sharp chest pain when you breathe in, or fainting alongside a swollen leg, that combination is a medical emergency.
Doctors typically use ultrasound to check for a clot. For clots in the upper leg and thigh, ultrasound catches about 96.5% of cases. It’s less reliable for clots isolated to the calf, where sensitivity drops to around 57%, though false positives are rare in either location.
Cellulitis and Other Infections
Cellulitis is a bacterial skin infection that causes one-sided swelling along with pain, redness, and skin that feels warm and tender to the touch. The skin may look pitted, almost like the surface of an orange, and blisters can form in the affected area. Some people develop fever and chills as the infection spreads deeper.
Cellulitis typically enters through a break in the skin: a cut, a crack between the toes, an insect bite, or even dry, cracked skin from conditions like athlete’s foot or eczema. If the area of redness is spreading quickly or you develop a fever, that signals the infection is advancing and needs prompt treatment with antibiotics.
Chronic Venous Insufficiency
The veins in your legs contain tiny one-way valves that push blood back up toward the heart. When those valves weaken or get damaged, blood pools in the lower leg, causing swelling that tends to worsen through the day and improve overnight. This is called venous insufficiency, and it often affects one leg more than the other.
The most common reason for it to be noticeably worse on one side is a prior DVT in that leg. Even after the clot itself resolves, the inflammation it caused can scar the vein valves, leaving permanent damage. Another structural cause is May-Thurner syndrome, where the left iliac vein (in the pelvis) gets compressed by an overlying artery, restricting blood flow out of the left leg specifically. Over time, venous insufficiency can cause skin discoloration around the ankles, varicose veins, and in advanced cases, open sores on the lower leg.
Lymphedema
Your lymphatic system acts as a drainage network, clearing excess fluid and proteins from your tissues. When that system is blocked or damaged on one side, fluid accumulates and the leg swells. This is lymphedema, and it has a distinct feel: early on, the swelling is soft and doughy, but over months or years, the skin thickens, becomes firm, and develops a rough texture.
The most common cause in developed countries is prior surgery or radiation therapy, particularly for cancers that involve removing lymph nodes in the groin or pelvis. The onset can be surprisingly delayed. Published case reports describe lymphedema appearing decades after the original procedure, suggesting that the lymphatic system can compensate for a long time before tipping into visible swelling. A minor injury or infection in the leg can be enough to overwhelm a compromised system and trigger the first episode.
One useful clue: if you press a finger into the swollen area and the dent stays for several seconds, that’s called pitting edema, which is typical of blood clot-related or venous swelling. Lymphedema in its later stages tends to be non-pitting, meaning the skin springs back and doesn’t hold a dent. Early lymphedema, however, can pit just like venous swelling, so this test alone isn’t enough to distinguish between them.
A Ruptured Baker’s Cyst
A Baker’s cyst is a fluid-filled sac that forms behind the knee, usually in people with arthritis or a knee injury. Most of the time it’s harmless, but if it ruptures, synovial fluid from the joint escapes into the calf muscles and triggers a strong inflammatory reaction. The result is sudden calf pain, swelling, and tightness that can look almost identical to a DVT.
This overlap is common enough that it has its own name: pseudo-thrombophlebitis. Even the physical exam findings can be misleading, since forced ankle movement reproduces pain in both conditions. The standard approach is to do an ultrasound first to rule out a blood clot, and if the veins look normal, follow up with imaging focused on the knee and calf to confirm the ruptured cyst. Ruptures typically happen after sudden physical activity, a quick knee bend, or direct impact to the back of the knee.
Other Causes Worth Knowing
A few less common causes can also explain one-sided leg swelling:
- Muscle strain or injury. A torn calf muscle or significant bruise causes localized swelling, usually with a clear connection to an activity or event.
- Lipedema. This is a condition involving abnormal fat distribution, primarily in the legs, that causes non-pitting swelling. It almost always affects both legs symmetrically but can occasionally appear more prominent on one side.
- Compartment syndrome. After a severe injury, swelling within the tight muscle compartments of the lower leg can build pressure to dangerous levels. This causes intense pain that seems out of proportion to the injury and is a surgical emergency.
How Doctors Figure Out the Cause
The first thing a clinician does is press on the swollen area to check whether the swelling pits or not. Pitting edema suggests a venous problem, such as a blood clot, venous insufficiency, or early lymphedema. Non-pitting swelling in a thickened, firm leg points toward later-stage lymphedema.
From there, the workup depends on the clinical picture. For suspected DVT, doctors use a scoring system that weighs factors like recent immobility, prior clots, cancer history, heart rate, and whether the leg is painful to touch. A high score means imaging is needed right away. A very low score in someone under 50 with a normal heart rate and no risk factors can sometimes rule out a clot without any imaging at all.
Ultrasound is the go-to test for most cases of one-sided leg swelling. It can confirm or rule out a blood clot, identify venous insufficiency, and detect a Baker’s cyst. For suspected compression of veins deeper in the pelvis, CT or MRI scans provide a clearer view of the anatomy.
Patterns That Help You Tell the Difference
Timing is one of the most useful clues. Swelling that appeared suddenly over hours, especially with calf pain and warmth, raises concern for a blood clot or ruptured cyst. Swelling that came on over days with spreading redness and fever suggests an infection. Swelling that has been gradually worsening over weeks or months points toward venous insufficiency or lymphedema.
Location matters too. DVT swelling typically starts below the knee and can extend up the thigh. Cellulitis often has a visible border of redness that you can almost trace with a finger. Lymphedema tends to involve the foot and toes (the toes look squared off and puffy), while venous swelling often spares the foot and concentrates around the ankle and calf.
Pay attention to what makes it better or worse. Venous swelling improves when you elevate your leg overnight and worsens after standing all day. Lymphedema is less responsive to elevation alone. A blood clot causes persistent swelling and pain that doesn’t come and go with position changes in the same predictable way.