The aim of this study was to establish a method to predict the antiplatelet effects of aspirin in vivo based on in vitro results. Aspirin in 5 different concentrations was added to the platelet-rich plasma samples, and the rates of platelet aggregation induced by collagen were determined in vitro. In addition, platelet aggregation and plasma drug concentration values were determined in vivo before and after the administration of aspirin (162 mg). The 50% effective concentration (EC50) values obtained from the in vivo and in vitro experiments were shown to have relevance, because the EC50 ratio for each subject was the same (0.23 ± 0.03). The actual and predicted values for the rate of inhibition of platelet aggregation were well correlated (P < .0001, r = .95) when the predicted rate was determined using the present method. Our results suggest that the antiplatelet effects of aspirin can be predicted using blood samples obtained before its administration.