Klippel-Trenaunay-Weber (KTW) syndrome: A congenital malformation syndrome characterized by the triad of asymmetric limb hypertrophy, hemangiomata, and nevi. "Asymmetric limb hypertrophy" is enlargement of one limb and not the corresponding limb on the other side, the enlarged limb being 3 times more likely to be a leg than an arm in KTW; and the limb enlargement is of bone as well as soft tissue. The hemangiomas, abnormal nests of blood vessels that proliferate inappropriately and excessively, cover a remarkable range from small innocuous capillary hemangiomas ("strawberry marks") to huge cavernous hemangiomas. The nevi are pigmented moles on the skin; in KTW there are often also dark linear streaks on the skin, streaks due to too much pigment. There can be other abnormalities but the triad is the consistent clinical centerpiece of the disease. Most persons with KTW have an enlarged leg and do relatively well without treatment or, for example, with only compression from an elastic stocking. Skin ulcers and other skin problems can occur over the swollen leg. Usually, the treatment is conservative. Surgery is almost never needed. The only possible exceptions are the very rare situations in which the leg reaches gigantic proportions or secondary clotting difficulties arise (due to trapping and destruction of blood platelets in a huge hemangioma). Then, amputation may become necessary. The cause of KTW syndrome is unknown.