Forty-nine patients with the Klippel-Trenaunay syndrome have been studied. Sixty-eight per cent have a superficial, embryological venous channel on the lateral aspect of the limb, 25 per cent have had one or more severe spontaneous haemorrhages from dilated varices and 22 per cent have suffered a venous thrombo-embolism. Twenty-nine per cent have had episodes of rectal bleeding or haematuria associated with pelvic angiomas. Foot volumetry (n = 31) showed that calf emptying was decreased in only five patients and only one of these had phlebographic evidence of deep venous obstruction, which conflicts with previous findings. The rate of calf refilling was increased in 81 per cent and phlebography demonstrated incompetent communicating veins in 45 per cent of patients. No patient had clinical evidence of an arteriovenous fistula, and arteriography (n = 22) and calf blood flow (n = 33) were normal, but lymphangiography (n = 14) showed lymphatic hypoplasia in 55 per cent of limbs. Eighty-eight operations have been performed on 38 patients but symptoms persist in 90 per cent. We suggest that surgery should be limited to the excision of localized symptomatic abnormalities and that the best form of control is provided by graduated compression stockings.