The antiplatelet agents, aspirin and ticlopidine, are widely used to prevent thromboembolism following cardiac valve replacement. To compare the clinical effects of each platelet inhibitor, a daily dose of ticlopidine 300 mg was given to 50 patients who underwent aortic valve or mitral valve replacement with an average age of 56.9 years over a mean 52.6 months after surgery. 50 more patients with an average age of 50.2 years were given a daily dose of aspirin 81 mg over a mean 51.3 months after surgery. Warfarin was given to maintain thrombotest values at 10 to 25% (PT-INR at 1.6-3.0). The incidence of thromboembolism was low in both groups; 1.0/100 patient years in the ticlopidine group and 1.9 in the aspirin group. Hemorrhagic complications, hematuria and ecchymosis, showed an incidence of 2.9 in the ticlopidine group and 2.3 in the aspirin group. Slight increases in GOT and GTP were observed in 4 and 18% of cases and elevated total cholesterol and neutral fat in 2 and 18% of cases. No adverse reactions were reported. With the exception of a significant decrease in ADP-induced platelet aggregation in patients who took ticlopidine, there were no significant differences observed between the two groups.