The effects of ticlopidine, a new antiplatelet agent, on graft patency rate was investigated in a group of 150 consecutive patients undergoing aortocoronary bypass graft procedures. In a double-blind study, two groups of 75 patients each received ticlopidine (250 mg twice daily) or a placebo during a three-month period. Graft patency was evaluated by repeat angiography in 38 patients and by rest and stress myocardial scintigraphy in 93. Combined angiographic and scintigraphic analysis was performed in 36 patients. The biological effects of ticlopidine on platelet activity were assessed by bleeding time and appropriate ex vivo tests. Graft patency was evaluated in 131 patients (87%). Evaluation was performed at the end of the three-month therapy period in 77 patients (Groups T1 [ticlopidine] and P1 [placebo]) and five months later in 54 patients. The total graft attrition rate was 10.1% in Group T1 compared with 20.3% in Group P1 (p less than 0.1). Excluding patients with discordant biological data, the attrition rate was 7.1% for Group T1 compared with 21.8% for Group P1 (p less than 0.02). There was no difference between the subgroups evaluated five months after the end of therapy. Ticlopidine appears to be an effective means of protecting graft patency as long as the biological effects of the drug are present. This protective effect disappears when the drug is discontinued, which suggests that ticlopidine should be prescribed for a longer period, at least for the first year after operation.