Background: Advanced age, major orthopedic surgery, neoplastic disease, prolonged operations, varicose veins, immobilization, estrogen-containing medications, and obesity are known risk factors for the development of postoperative thromboembolic complications. Perioperative heparin is useful for reducing the incidence of deep venous thrombosis (DVT), but it is associated with a discrete bleeding rate. The purpose of this study was to determine the incidence of clinically evident DVT in morbidly obese patients after laparoscopic Roux-en-Y gastric bypass when a pneumatic compression hose is used as the only prophylaxis against DVT instead of anticoagulants. Methods: From April 2000 to April 2003, 380 patients underwent laparoscopic Roux-en-Y gastric bypass for morbid obesity by one surgeon (R.H.C.). Prospectively, each patient was clinically evaluated for the presence of DVT during the postoperative period. Calf-length pneumatic compression stockings were placed before the procedure began, and remained in place until the patient was ambulatory. Ambulation was encouraged on the evening of the operation. No pharmacologic anticoagulant was used as a prophylaxis against DVT. Results: Of the 380 patients, 346 were women and 34 were men with a mean age of 39.3 ± 9.4 years (range, 14-65 years). The mean weight of these patients was 299.5 ± 53.6 lb (range, 188-483 lb), and their mean body mass index was 48.5 ± 6.6 (range, 36-70). The mean operative time was 103.3 ± 24.3 min (range, 62-227 min), and mean American Society of Anesthesiology (ASA) score was 2.6. Nine patients had clinical evidence of severe, chronic venous disease preoperatively. One patient (0.26%) experienced a clinically evident DVT limited to the popliteal vein on duplex ultrasonography. The clot resolved completely, as evidenced by follow-up duplex ultrasonography after 2 weeks of subcutaneously injected fractionated heparin. No clinically evident pulmonary thromboembolism occurred in this study group. Conclusions: The incidence of clinically evident DVT after laparoscopic Roux-en-Y gastric bypass is low when the procedure is accomplished with a relatively short operative time, with the initiation of calf-length pneumatic compression hose before the induction of anesthesia, and with routine early ambulation. No form of heparin anticoagulation is mandatory when these conditions can be met.