Swelling in one leg without a history of trauma can be attributed to deep vein thrombosis or lymphedema, according to the Journal of the American Board of Family Medicine. Deep vein thromboses require immediate medical intervention, and lymphedema can become a serious medical condition if not appropriately treated.
Per the U.S. National Library of Medicine, deep-vein thromboses can occur at any age and typically affect one of the legs; symptoms include reddening of the skin, pain, swelling and warmth of the area. DVTs occur when a large vein is blocked by a blood clot; when a DVT breaks loose from the wall of the vessel it is blocking and moves through the bloodstream, it is called an embolus and can cause potentially lethal damage to the heart, lungs and brain.
According to the National Lymphedema Network, lymphedema is a collection of protein-rich fluid resting just below the surface of the skin. Lymphedema commonly occurs in the legs, and it can be caused by damage to lymph vessels, removal of lymph nodes or genetic conditions. This condition is progressive if it is not treated appropriately, and it can lead to complications like local tissue fibrosis, delayed wound healing and increased risk of infection.