Two swollen legs is usually a slow story. One swollen leg is a different question entirely, because the explanations are local — and one of them is time-sensitive.
If one leg has newly become swollen
Be seen today — emergency department, urgent care, or same-day primary care, wherever can arrange an ultrasound now. This is especially true if it came on over hours to days, or comes with pain, calf tenderness, warmth or a change in skin color.
And call 911 for sudden breathlessness, chest pain that is worse on breathing in, coughing blood, a racing heart, or fainting — those can mean a clot has reached the lungs.
Why asymmetry raises the stakes
Systemic causes of swelling — the heart, the kidneys, the liver, a medication — act on the whole body, so they swell both legs. When one leg is swollen and its neighbor is not, something is happening in that leg or in the drainage path leaving it. That narrows the list to local causes, and deep vein thrombosis sits on it.
The other reason asymmetry matters: you have a built-in comparison. Two legs that have aged together make any difference between them meaningful in a way a single measurement never is.
What can cause it
| Cause | Typical picture | Urgency |
|---|---|---|
| Deep vein thrombosis | Swelling over hours to days, often calf pain or tenderness, warmth, color change — or no symptoms beyond the swelling | Today |
| Cellulitis | Hot, red, tender, spreading; usually fever and feeling unwell | Today |
| Ruptured Baker’s cyst | Sudden calf pain and swelling, often a history of knee problems; can mimic a clot closely | Needs assessment to distinguish from DVT |
| Injury | A clear event, bruising, pain on movement | Depends on severity |
| Venous insufficiency on that side | Gradual, years, worse by evening, visible veins or skin staining | Routine |
| Post-thrombotic syndrome | Chronic swelling in a leg that had a clot in the past | Routine |
| Lymphedema | Firmer swelling involving the foot and toes, less overnight improvement | Routine, but new onset needs a cause looked for |
| Compression from a mass | Progressive swelling with no other explanation | Needs investigation |
| After surgery or radiotherapy in the pelvis or groin | Swelling appearing weeks to years later on one side | Routine, with assessment |
The question we get asked, and the honest answer
“Is there a way to tell whether it is a clot?” No — not from home, and not from examination alone in a clinic either. Around half of deep vein clots cause no symptoms, and those that do look like several other things. Clinicians use a structured risk score, a blood test and an ultrasound precisely because looking at the leg does not settle it. That is why the advice here is a plain instruction to be seen rather than a checklist.
What to tell whoever sees you
- When it started, as precisely as you can — hours, days, weeks or months.
- Whether anything preceded it: surgery, a long flight or drive, an injury, illness, a hospital stay, a cast.
- Whether it hurts, and where — calf, whole leg, foot.
- Whether you have had a clot before, or anyone in your family has.
- Your medicines, including contraception and hormone therapy.
- Whether you have cancer or are having treatment for it.
- Any breathlessness or chest discomfort, however mild — say this first.
What happens next
Usually a clinical assessment combining your symptoms and risk factors, often a D-dimer blood test, and a compression ultrasound of the leg veins. If a clot is found, anticoagulation starts. If it is not, attention turns to the other causes on the list above — and “not a clot” is a useful result even when it leaves the swelling unexplained, because it removes the time pressure.
Common questions
It is not very swollen. Can I wait until Monday?
Severity is a poor guide here. A clot can produce mild swelling, and a dramatic-looking leg can be a ruptured cyst. What raises the priority is that it is one leg, that it is new, and that a clot is on the list — not how bad it looks.
Should I elevate it or walk on it while I wait?
Elevate it if it is comfortable, and do not massage or vigorously stretch the calf. Do not start compression that nobody has assessed you for. Mostly, do not spend the waiting time on remedies — spend it getting seen.
My leg has been swollen for years, only on one side. Same urgency?
No. Longstanding, stable, one-sided swelling with a known explanation is a routine matter. What needs same-day attention is new swelling, or a longstanding leg that has changed noticeably.
Could it be an insect bite or an allergy?
Possibly, and those usually come with a visible lesion, itch and a shorter story. The problem is that cellulitis can start that way too, and a clot does not care what else is going on in the leg. A swollen leg that is also hot and spreading is a same-day matter whatever started it.
Where to go next
Sources
- Centers for Disease Control and Prevention. About Venous Thromboembolism (Blood Clots).
- NHS. Deep vein thrombosis (DVT).
- NHS. Oedema.
- National Heart, Lung, and Blood Institute. Venous Thromboembolism.
Written by Tom Biernacki, DPM, FACFAS. Published 20 September 2026. General education, not medical advice.