A prospective, double-blind clinical study was performed to evaluate the combination of dipyridamole 225 mg/day and acetylsalicylic acid 1 gm/day prophylaxis of postoperative venous thromboembolism in elective total hip replacement. Patients were stratified according to age, and randomly assigned to receive drug or placebo. All patients were followed with 125I-labeled fibrinogen scanning for one week postoperatively, or until fully mobile. Venography was performed in 79/132 patients; in 36 patients the venogram was obtained to confirm a positive fibrinogen scan, in 43 patients an elective venogram was obtained on the seventh postoperative day to evaluate the operated thigh (a blind area for scanning). Thrombosis (by scan or venogram) was found in 17/68 (25%) in the control group, and in 23/64 (36%) in the treated group. Overall incidence was 40/132 (30%). Correlation of scan with venography was 90%. There were no clinically significant pulmonary emboli in either group. The combination of acetylsalicylic acid (ASA) and dipyridamole as given in this study is not effective prophylaxis in elective total hip replacement.