(1) Over a period of ten years, a series of 113 patients had prosthetic valve replacement, of which 83 patients were consisted of 44 aortic replacements, 31 mitral, 5 tricupid, 3 combined. The frequency of thromboembolism was studied in patients receiving warfarin potassium with dipyridamole (225 mg/day); warfarin; dipyridamole; and in those neither receiving warfarin nor dipyridamole. In aortic group, there were no significant incidence of thromboembolic complications with each group followed-up for average 53 months. But in mitral "warfarin with dipyridamole" group revealed significantly reduced incidence of thromboembolic deaths in comparison with "not receiving anticoagulants" group followed-up for average 62 months (n=20, t=2.3041, p<0.05). And there were no other significant correlation with use or without use of anticoagulants. (2) The effect of dipyridamole on platelet function was studied in twenty-three patients who had been implanted prosthetic valves. All patients were received an anticoagulant therapy of warfarin. Platelet adhesiveness was estimated by modification of Salzman's method and platelet aggregation was measured with turbidometric method by Born and O'Brien. Before administration of the drug, 9 patients of aortic group revealed no significant change in platelet adhesiveness and also in platelet aggregation. Platelet adhesiveness remained unchanged by dipyridamole administration up to the dosage of 450 mg/day. But aggregation receiving dipyridamole (300 mg/day to 450 mg/day) was statistically reduced in comparison with normal control in % aggregation (p<0.05). There were no significant difference with from 150 mg/day group to 450 mg/day group with each other. On the other hand, in 14 patients of mitral group, the platelet adhesiveness receiving dipyridamole (150 mg/day) was increased statistically (p<0.02). The platelet adhesiveness was, however, by 225 mg/day of dipyridamole reduced to a mean normal retention. But further ingestion of 450 mg/day, made no obvious change. And studies on platelet aggregation treated with dipyridamole of from 150 mg/day to 225 mg/day, showed significantly shortened RT (reaction time to the maximum aggregation, p<0.005) and b/2 T(reaction half-time to the maximum aggregation, p <0.05-0.005). After administration of 375 mg/day of dipyridamole,
the platelet aggregation curve showed the prolongation of RT and b/2 T to a normal mean of aggregation, and further administration of dipyridamole 450 mg/day revealed significantly reduced % agg. in comparison with normal control (p<0.05). (3) From my study, in aortic group, there is no necessity of anticoagulants of warfarin, but dipyridamole 300 mg/day which influences the platelet behavior, is advisable. But in mitral group, anticoagulants of warfarin is necessary with combined use of dipyridamole 450 mg/day.