The aim of this study was to investigate prospectively the incidence of deep venous thrombosis (DVT) after surgery for morbid obesity. The series comprised 116 consecutive patients undergoing Roux-en-Y gastric bypass. The median age and body mass index were 35 years (range 19-59 years) and 42 kg/m2 (range 32-68 kg/m2), respectively. The patients were examined with duplex ultrasonography pre- and postoperatively. No patient had any symptoms or signs of DVT postoperatively, and ultrasonography showed no signs of thrombosis in iliac, femoral, and popliteal veins in any of the patients. Two patients (1.7%) had a thrombus in the peroneal vein of one leg. Repeated ultrasonographic investigation after 1 week showed complete resolution of both. One patient with a previously unknown activated protein C resistance had an angiographically confirmed minor pulmonary embolus. The incidence of venous thromboembolism after obesity surgery seems to be low, and obesity as a risk factor for thromboembolic disease might have been overestimated in the past.