To assess whether aspirin reduces the risk of a first myocardial infarction (MI) in young women, we evaluated data from a case-control study among women less than 50 years of age without a prior MI: 48 of 551 cases of MI and 67 of 896 hospital controls had taken aspirin regularly for at least 12 weeks immediately before admission. The relative risk estimate was 0.8 upon allowance for confounding factors but it was not statistically significant (95 per cent confidence interval, 0.5-1.4). These data alone do not provide evidence of protection by aspirin against a first infarction in young women.