An 18-year-old woman presented with a large anterior myocardial infarction. Her cardiovascular risk factors were cigarette smoking in moderation and oral contraception with a synthetic oestroprogestative pill prescribed a few months previously. Coronary angiography showed occlusion of the left anterior descending artery but no other lesions. Biological investigations excluded an abnormality of coagulation. Antibodies to synthetic steroids (ethinylestradiol and progesterone) and circulating immune complexes were found in the serum. The role of antiethinylestradiol antibodies in the mechanism of myocardial infarction is discussed. These antibodies are present in 30 per cent of women taking oral contraceptives and their titres are significantly higher in 90 per cent of women who develop vascular thrombosis unrelated to atherosclerosis. The mechanism of the thrombogenic action of the antibodies and circulating immune complexes is also considered.