Veins are vessels through which blood travels from the body back to the heart. Blood tends to clot in veins in the leg, and complications depend on the vein location. Clots in veins closer to the surface of the leg (superficial venous thrombosis) are believed to cause discomfort and swelling but few other serious complications. Clots in veins deeper in the leg (deep venous thrombosis) can break off and travel to the lungs (pulmonary embolism), causing more serious symptoms or death. Researchers have increasingly recognized that people with 1 kind of clot may have both kinds. If people with surface leg clots also often have deeper leg clots, then surface clots may be more serious than is commonly recognized.