Based on a retrospective evaluation of 107 patients with congenital venous angiodysplasia of the Type Klippel-Trenaunay (n = 76) and Type Servelle-Martorell (n = 31) the frequency and pathogenesis of aneurysma formation in the venous system has been analysed. The vascular patterns include both cylindric ectasia and fusiform aneurysms with an incidence of approximately 40%. Preferred locations are subcutaneous drainage veins, the popliteal, external iliac vein and atypic communicating veins between the superficial and the deep venous system. Complications of the aneurysm such as local thrombosis, recurrent pulmonary embolism or bleeding from ruptur were not observed. The aneurysma formation in venous angiodysplasias results probably from the causative factors: congenital weakness of the venous wall and an abnormal hemodynamical stress situation. The latter is caused by concomitant malformations of the deep venous system. The persistent intermittent venous hypertension associated with a more or less pronounced increase of the venous volume in the affected venous system of the limb results in a deep venous insufficiency respectively venous reflux disease. The therapy of choice is predominantly conservative, i.e., external compression bandages or stockings to reduce the deleterious effects of a chronic deep venous insufficiency respectively venous reflux disease. Surgery is indicated under two conditions: a) in the presence of aneurysm complications or b) for the elimination of a pathological short circuit flow in some drainage veins. Antireflux surgery, i.e., venous valve transfer from the brachial vein, is up to recently still in a stage of experimental-clinical investigation.