We studied 345 cases taking aspirin due to increased platelet aggregation. 220 were male and 125 were female. They were divided into five groups (0, 30 mg, 50 mg, 300 mg, 500 mg, daily) with double-blind method. Platelet aggregation tests were performed before and after medication. The result shows that the response in male is good. For female, the benefit in menopausal women and contraceptive taker is more prominent than that in adolescent and middle aged women. From our clinical trial we suggest 50 mg of aspirin daily as the optimal dosage.