Antibodies to cardiolipin were measured in 62 survivors of myocardial infarction under age 45 at 3, 12, and 36 months after the acute event. 13 patients (21%) had raised anticardiolipin antibody levels on at least two of the three sampling occasions. Risk-factor profiles and coronary angiographic findings did not differ between the anticardiolipin-positive group and the rest of the patients. No correlation was found between cardiolipin and anti-DNA antibody levels. 8 of the 13 patients with raised anticardiolipin antibody levels experienced additional cardiovascular events during a follow-up of 36-64 months after the first myocardial infarction: cerebral infarction developed in 2, arterial occlusion of the lower limb in 2, new myocardial infarction in 3, pulmonary emboli in 1, and deep-vein thrombosis in 1. These 8 patients had cardiolipin antibody titres of 5 times the mean for voluntary blood donors. Antibodies to cardiolipin are common in young post-infarction patients and should be interpreted as markers of high risk for recurrent cardiovascular events.