A randomized prospective study of low dose heparin was performed in 175 surgical patients to determine the frequency of bleeding and wound complications. The patients were divided into three groups: (1) low dose heparin (5000 units two hours before operation and 5000 units every 12 hours following operation for five days); (2) low dose heparin postoperatively only; and (3) a control group. The frequency of bleeding and wound complications was 27% in group I, 7.5% in group II, and 1.4% in group III. The difference between the control patients and those heparinized pre- and postoperatively is statistically significant (p less than 0.005). None of the patients in any of the three groups had a pulmonary embolus, but the number of patients involved is too small to assess the significance of this finding. However, a bleeding and wound complication rate of 27% is significant. These findings indicate that perhaps the routine use of low dose heparin should be reserved for those patients with preoperative factors indicating an increased risk from thromboembolism.