A case-control design was used. The case series consisted of women 18 to 44 years of age admitted for a documented first myocardial infarction to any of 100 collaborating hospitals in Massachusetts, Rhode Island, and Pennsylvania. Nurse interviewers administered standard structured questionnaires to cases and controls on drug use, race, religion, medical history and myocardial infarction risk factors. Information was collected on: cigarette smoking; histories of hypertension, diabetes mellitus, angina, abnormal plasma lipids; reproductive and menstrual histories; alcohol and coffee consumption; personality type using the Framingham Type A scale; family history of myocardial infarction; exercise; education and occupation of the patient and her spouse. Information was also obtained on the timing and duration of oral contraceptive use. The effect of oral contraceptive use was considered among nonsmokers, light smokers, and heavy smokers. Individual oral contraceptive formulations were classified according to dosage and type of estrogen, formulation type, and other groupings. Multiple logistic regression analysis with fitting of the regression equations by the method of maximum likelihood was used for simultaneous control of all potential confounding factors.