Swelling in one leg, specifically from the knee down, almost always points to a local problem rather than a whole-body condition like heart or kidney disease (which typically causes both legs to swell). The most common causes include a blood clot in a deep vein, an infection in the skin or soft tissue, chronic valve damage in the veins, lymphatic fluid buildup, or a ruptured cyst behind the knee. Because some of these are medical emergencies and others are slow-developing nuisances, identifying the cause matters a great deal.
The fact that it’s your left leg is worth noting. An anatomical quirk makes the left leg more vulnerable to certain types of swelling than the right.
Why the Left Leg Is More Vulnerable
In your pelvis, the right iliac artery crosses directly over the left iliac vein. This means the main vein draining your left leg is partially compressed by an artery sitting on top of it. In most people, this causes no issues. But in some, the chronic pressure narrows the vein enough to slow blood flow and raise the risk of clotting. This condition, called May-Thurner syndrome, is one reason blood clots form more often in the left leg than the right. It can also contribute to chronic swelling even without a full clot forming.
Blood Clots in the Deep Veins
A deep vein thrombosis (DVT) is the most urgent cause of one-sided leg swelling. A clot forms in the deep veins of the leg, blocking blood from returning to the heart efficiently. The leg swells, often feels tight or heavy, and may be tender along the inner thigh or calf. The skin usually looks normal or slightly red, and the surface feels smooth rather than bumpy or textured.
Your risk is higher if you’ve recently had surgery, been on a long flight or car ride, spent several days in bed, have active cancer, or have had a DVT before. A calf that measures more than 3 centimeters larger than the other leg is one of the clinical markers doctors use when scoring your likelihood of having a clot. Pitting edema, where pressing a finger into the swollen area leaves a temporary dent, is another sign that raises suspicion.
DVT matters not just because of the leg swelling but because pieces of the clot can break off and travel to the lungs. This is a pulmonary embolism, and it can be life-threatening. Warning signs include sudden sharp chest pain that worsens with deep breaths, unexpected shortness of breath even at rest, a rapid heartbeat, coughing (sometimes with blood), dizziness, or fainting. If you have leg swelling along with any of these symptoms, that combination requires emergency care.
Cellulitis and Skin Infections
Cellulitis is a bacterial infection of the skin and tissue beneath it. It causes swelling that looks and feels different from a blood clot. The skin turns red, feels hot to the touch, and often develops a dimpled, orange-peel texture. You may see red streaks radiating outward from the swollen area, and nearby lymph nodes (behind the knee or in the groin) may become swollen and tender. Fever is common.
With a DVT, by contrast, the skin surface stays smooth, the temperature is normal or only mildly warm, and you won’t have swollen lymph nodes or red streaks. This distinction can help you describe your symptoms more precisely, though both conditions need professional evaluation. Cellulitis can worsen rapidly, spreading deeper into tissue within hours if untreated.
Chronic Venous Insufficiency
If your swelling has been building gradually over weeks or months rather than appearing suddenly, damaged vein valves may be the culprit. Your leg veins contain one-way valves that push blood upward against gravity. When those valves weaken or fail, blood pools in the lower leg. The pressure builds throughout the day, causing swelling that’s worst by evening and improves overnight when you’re lying flat.
Over time, venous insufficiency progresses through recognizable stages. Early on, you may notice spider veins or varicose veins with mild ankle swelling. As it advances, the skin around your ankles may darken, become leathery or dry, and eventually break down into open sores called venous ulcers. This progression can take years, but it doesn’t reverse on its own. Venous insufficiency can affect one leg or both, but it’s frequently asymmetric, with one side significantly worse.
Lymphedema
Your lymphatic system is a network of vessels that drains excess fluid from tissues back into the bloodstream. When those vessels are damaged or blocked, fluid accumulates and the limb swells. Lymphedema in a single lower leg can develop after surgery that removed lymph nodes (common in cancer treatment), after radiation, or after repeated skin infections that scar the lymphatic channels.
Lymphedema feels different from other types of swelling. In the early stages it’s soft and pitting, similar to fluid retention. As it progresses, the tissue becomes firm and fibrotic, and pressing on it no longer leaves a dent. A useful self-check involves trying to pinch and lift the skin on top of your foot near the base of your second toe. If the skin is so thickened that you can’t pinch it into a fold, that suggests lymphedema. This test has a sensitivity of about 92%, meaning it catches most true cases, though a negative result doesn’t completely rule it out.
A Ruptured Baker’s Cyst
A Baker’s cyst is a fluid-filled sac that forms behind the knee, usually as a result of arthritis or a cartilage tear. Many are small and painless. But if the cyst fills too quickly or under too much pressure, it can burst, releasing fluid into the calf. This causes sudden, sharp pain in the back of the knee or upper calf, followed by swelling that spreads down the lower leg. Some people describe a sensation like water running down the inside of their leg.
A ruptured Baker’s cyst can closely mimic a DVT, with similar calf swelling and tenderness. Because the two conditions can look nearly identical and a DVT is far more dangerous, imaging is typically needed to tell them apart.
How Doctors Figure Out the Cause
When you show up with a swollen leg, the evaluation usually starts with your history and a physical exam. Doctors assess your risk factors for a blood clot using a scoring system that adds points for things like recent surgery, immobilization, cancer treatment, prior clots, and specific physical findings like pitting edema and calf asymmetry. A score of 2 or higher places you in a moderate-to-high probability category for DVT.
A blood test that measures a protein fragment released when clots break down is often the next step. This test is extremely good at ruling out clots, with a sensitivity up to 97%. If the result is normal in someone with a low risk score, a DVT is very unlikely. However, the test has poor specificity (as low as 35%), meaning many things besides clots can make it positive: infection, inflammation, recent surgery, pregnancy. So a positive result doesn’t confirm a clot. It simply means imaging, usually an ultrasound of the leg veins, is the next step.
For suspected lymphedema, specialized imaging can track how well lymphatic fluid moves through your leg. For venous insufficiency, an ultrasound can visualize valve function in real time, showing whether blood flows backward when you stand.
Managing Swelling at Home
Until you know the cause, elevation is the safest first step. Lying down with your leg raised above heart level helps fluid drain regardless of the underlying problem. Avoid sitting or standing in one position for long stretches.
If your swelling has been diagnosed and you’ve been advised to use compression stockings, the pressure level matters. Graduated compression stockings come in several grades:
- 15 to 20 mmHg (mild): Suitable for very early or minimal swelling, prevention during travel, or building tolerance to compression.
- 20 to 30 mmHg (moderate): The most commonly prescribed level for mild to moderate swelling from venous insufficiency or early lymphedema after treatment.
- 30 to 40 mmHg (firm): Used for more established venous disease, lower-leg lymphedema with tissue thickening, or cases where moderate compression isn’t controlling the swelling through the day.
- 40 to 50 mmHg and above: Reserved for severe lymphedema with significant tissue changes, used only with clinical guidance.
The lower leg requires more compression than the arm to overcome gravity, so many people who start at 20 to 30 mmHg find they need 30 to 40 mmHg for adequate control. Compression should not be used if there’s any concern about arterial blood flow problems, so getting the right diagnosis first is important.
Patterns That Help Identify the Cause
The timeline and character of your swelling offer useful clues. Sudden onset over hours, especially with calf pain and tightness, points toward a blood clot or ruptured cyst. Swelling accompanied by red, hot, dimpled skin and fever suggests infection. Gradual worsening over weeks or months, particularly if it improves with overnight rest, is more consistent with venous insufficiency. Swelling that started after surgery or cancer treatment and no longer pits when pressed suggests lymphedema.
One-sided swelling from the knee down that appeared without an obvious injury is not something to watch and wait on. The range of possible causes spans from manageable chronic conditions to medical emergencies, and the early symptoms can overlap enough that clinical testing is the only reliable way to sort them out.