Since 1945, we have operated on 786 patients with Klippel and Trénaunay's syndrome. Elongation of the impaired limb was invariably found while edema was present in 84%, varicose veins in 36%, and flat angiomata in 32%. Venography and surgical exploration have demonstrated malformation of the deep veins involving the popliteal vein in 51%; superficial femoral vein, 16%; both popliteal and superficial femoral veins; 29%; iliac veins, three per cent; and lower vena cava, one per cent. Good clinical results have been achieved following the surgical release of these deep veins in the lower limb. During childhood, when the difference in limb length is noteworthy, ligature of the popliteal vein of the shorter limb induces a compensating elongation. Klippel and Trénaunay's syndrome may be associated with lymphatic malformations, including lymphedema and malformation of the lymph vessels. Knowledge of the pathophysiology of these malformations of the deep veins enables a better understanding of the clinical manifestations of the condition, as well as the improved treatment of the serious vesical or rectal hemorrhage which occurs in one per cent of these patients.