The clinical and laboratory effects of low-dose heparin prophylaxis was prospectively studied in a controlled trial of 182 patients undergoing major surgery for gynecologic malignancy. Low-dose heparin was given in 5000 U subcutaneously two hours preoperatively and every 12 hours for seven days postoperatively. Low-dose heparin-treated patients had a significantly increased daily retroperitoneal hemovac drainage. Although not statistically significant, low-dose heparin was associated with increased estimated intraoperative blood loss, transfusion requirements, and wound hematomas. Fifteen percent of patients receiving low-dose heparin were found to have an activated partial thromboplastin time greater than 1.5 times the control value. In these patients, all clinical bleeding parameters were significantly increased. Low-dose heparin-treated patients also had significantly prolonged activated partial thromboplastin time and lower final platelet counts as compared with the control patients. When using low-dose heparin for thromboembolism prophylaxis, patients should be closely observed for clinical hemorrhagic complications. Activated partial thromboplastin times and platelet counts should be monitored throughout therapy.