This was a case-control study. The computerized files of the Group Health Cooperative (GHC) in Seattle were used to identify cases. All postmenopausal women, aged 40 to 79, were eligible as cases if, according to World Health Organization criteria, they presented with a fatal or non-fatal myocardial infarction. A random sample of women listed in the enrollment files at GHC served as a source of potential controls. Review of the out-patient medical records, performed by an assistant blind to case-control status ensured that all study subjects met the same criteria. Telephone interviews were used to obtain information about exposure to postmenopausal hormones as well as known risk factors. The computerized pharmacy records at GHC provided additional information about exposure. Frequency matching controlled for the potential confounding effects of age and year of presentation. Stratification and logistic regression were used in data analysis.