The etiology of the Klippel-Trenaunay syndrome (KTS) remains obscure. Although venous hypertension secondary to deep venous obstruction has been suggested as a cause, recent studies have demonstrated that most patients have unimpeded venous drainage. Calf blood flows have been measured in 33 patients with KTS using venous occlusion plethysmography. Although all flow rates were within normal limits, flow in affected limbs was invariably greater than in normal limbs (p less than 0.001), and this is related to the presence of the nevus. Biopsies of subcutaneous veins demonstrate the histological features of a response to chronically raised flow. The authors suggest that KTS is caused by mesodermal abnormality during fetal development, leading to the maintenance of microscopic arteriovenous communications in the limb bud, as a result of which the triad of nevus, hypertrophy, and superficial varices is produced. Deep venous abnormalities occur pari passu with the triad and are not responsible for its development.