Why Does Only One Leg Swell? Causes Beyond Blood Clots

One leg is swollen, the other isn't. It's natural to worry about a blood clot first — and that's worth ruling out — but it's far from the only explanation for one-sided leg swelling.

Why One-Sided Swelling Gets Attention

Unlike swelling in both legs, which usually points to something affecting your whole body, swelling in just one leg points to something local — happening specifically in that leg. That includes deep vein thrombosis, but it also includes several other causes that are more common and less serious.

Chronic Venous Insufficiency

Weakened vein valves that don't close properly, letting blood pool. Usually gradual, worse by evening, and can affect one leg more than the other if that leg's veins are more affected. Covered in more detail in our dedicated article on venous insufficiency.

Baker's Cyst

A fluid-filled swelling behind the knee, sometimes from an underlying joint issue. Can cause calf swelling if it ruptures, occasionally mimicking DVT symptoms closely enough that ultrasound is needed to tell them apart.

Cellulitis (Skin Infection)

A bacterial skin infection causing localized swelling, warmth, and redness — genuinely similar in appearance to DVT, which is part of why doctors don't rely on symptoms alone. Usually comes with more visible skin changes and sometimes fever.

Injury or Muscle Strain

A sprain, strain, or muscle tear causes localized swelling tied to a specific movement or impact, usually with a clear story behind it ("I twisted my ankle" or "I felt something pull").

Lymphatic Obstruction

Damage or blockage in the lymphatic drainage of one leg — from prior surgery, radiation, or in rarer cases a growth compressing lymphatic vessels — can cause persistent one-sided swelling that doesn't fit the "worse by evening, better by morning" venous pattern.

Deep Vein Thrombosis

The cause that needs to be ruled out promptly: sudden onset, paired with warmth, redness, and a constant ache. This is why one-sided swelling with any of these accompanying signs warrants same-day medical attention rather than a wait-and-see approach.

How Doctors Actually Tell These Apart

Symptoms alone aren't reliable enough to distinguish between these causes with confidence — that's exactly why ultrasound and clinical examination exist. The goal of knowing these categories isn't to self-diagnose; it's to know how urgently to get checked.

Vascular Surgeon's Take

The question that separates a Baker's cyst from most other causes on this list is where exactly the swelling starts — a cyst-related swelling begins behind the knee and tracks down into the calf, while venous or clot-related swelling typically starts at the ankle and works its way up. Asking a patient to point to where it started first, before it spread, narrows things down faster than almost any other question I ask

The Bottom Line

One-sided leg swelling has several possible explanations, and a blood clot is one of the more serious but not the most common. The safest approach is the same regardless of which cause turns out to be true: new, one-sided swelling — especially with pain, warmth, or redness — deserves a same-day evaluation rather than guessing.

Recommended Reading
One Swollen Leg vs. Two: What Your Body Is Telling You
Calf Cramp or Blood Clot? How to Tell the Difference in 60 Seconds