To the Editor. —There have been many studies relating the effect of aspirin prophylaxis on thromboembolisms. 1,2 Evaluation of the clinical data presented in them indicates that thromboembolic disease developed more rarely in men than in women under the identical dosage of aspirin given for prophylaxis against thromboembolism after hip surgery and transient ischemic attack. This can be explained on the following basis. The effect of aspirin on platelet aggregation is mediated by its acetyl group. The acetyl group of aspirin is essential for acetylation of cycloperoxygenase, 3 an enzyme that converts arachidonate to prostaglandin endoperoxide and forms thromboxane A 2 . By acetylation of the enzyme, it prevents formation of prostaglandin endoperoxide and thromboxane A 2 . These two substances are strong platelet-aggregating agents. For metabolism of aspirin, and to obtain the acetyl group, aspirin esterase is essential. In a study conducted by Windorfer et al, 4 it was observed that