If you see one leg swollen, it is often a sign of a localized problem rather than a whole-body issue. Common causes include injury or trauma (such as a sprain, fracture, or muscle tear), which can lead to inflammation and fluid buildup in the affected leg. Swelling may also appear after prolonged standing or sitting, especially if blood flow is reduced.

Another possible cause is infection, such as cellulitis, which usually comes with redness, warmth, pain, and sometimes fever. Infections can develop from small cuts, insect bites, or skin conditions and should be treated promptly to prevent complications.

A more serious concern is a blood clot, known as deep vein thrombosis (DVT). This typically causes swelling in one leg along with pain, warmth, or a feeling of tightness. DVT is a medical emergency because the clot can travel to the lungs and become life-threatening.

One-sided leg swelling can also be linked to circulation or lymphatic problems, such as venous insufficiency or lymphedema. If the swelling appears suddenly, worsens quickly, is painful, or is accompanied by chest pain or shortness of breath, medical care should be sought immediately.