The design was that of a case-control study. Cases were identified by weekly telephone calls to the coronary care units of 78 hospitals in greater Boston, Southern Connecticut, and Westchester County. Nurse-interviewers administered standard interviews to cases convalescing on medical wards after discharge from coronary care units and to controls identified in the same hospitals. Histories of oral contraceptive use, including the timing and duration of use and the name of the preparations, were recorded along with information on other drug use and myocardial infarction risk factors such as cigarette smoking. Data were collected for three years.