A hospital-based, case-control study of the association between current usage of oral contraceptives and first-time cases of acute myocardial infarction, stroke, or a venous thromboembolic event (deep vein thrombosis or pulmonary embolus) was carried out in 17 countries from four regions (Africa, Asia, Europe, and Latin America). A total of 3792 cases and 10,281 hospitalised controls matched for age were recruited during a 4-year period, ending in June 1993. The study was designed to have sufficient power to detect a relative risk of 2 for developing each of the three study diseases, associated with current oral contraceptive use in each of the four regions, with the exception of acute myocardial infarction (for which all non-European regions were to be combined) and for venous thromboembolic events in Asia, where these disorders are rare. This report describes the background, pilot study, methods, and the analyses carried out to validate the methods used in the study.