A total of 1,017 bypasses were performed in 442 patients operated on in our department between January 1, 1981, and May 30, 1984. The overall early postoperative graft patency rate in our hospital was 91.5%. About 10% of the grafts had a flow rate of 40 ml/min or less, measured intraoperatively, and most occluded grafts were in this group. This article presents our experience with low-flow bypasses whose patency rates we attempted to improve. Patients with aortocoronary bypasses (ACBs) and with intraoperative blood flow rates of 40 ml/min or less were divided into two groups. The treated group was given, from day 0 onward, a 500-mg dose of acetylsalicylic acid twice a day and a 75-mg dose of dipyridamole three times a day. The control group was given no medication. Control coronary arteriography was performed at one month and then at one year after the operation. One month postoperatively, 34 out of 41 ACBs in the treated group were patent; in the control group, only 17 out of 37 were patent (p less than .001). One year after the operation, 24 out of 37 ACBs in the treated group were patent, whereas in the control group only 8 out of 38 ACBs were patent (p less than .001). We conclude that antiplatelet drugs have a beneficial effect on the short-term and long-term patency of ACBs.