An increasing proportion of patients hospitalized with myocardial infarction have previously undergone coronary artery bypass surgery. To define this subgroup, 77 patients with acute infarction occurring 2 or more months (mean 52.8) after bypass surgery were compared with 77 control patients with infarction. Baseline characteristics of the groups were similar except that post-bypass patients were more often men (p = 0.02) and more likely to have had a previous infarction (37 versus 21, p = 0.008). Infarct size was smaller in the post-bypass group as assessed by peak creatine kinase (CK), peak CK-MB, maximal number of electrocardiographic leads with ST elevation, maximal summed ST elevation and QRS score measured 7 to 10 days after admission (p less than 0.001 for each variable). Five control patients but none of the post-bypass patients died in the hospital (p = 0.06). Serious complications (death, acute heart failure, ventricular fibrillation, second or third degree atrioventricular block) occurred in 24 control patients but in only 5 post-bypass patients (p less than 0.001). Angiography was performed after infarction in 45 of the 77 post-bypass patients. Occlusion of both a native coronary artery and its graft was found in 24 of the 45; these patients had had higher peak CK levels (p = 0.008) than the other 21 patients who had angiography. The probable causes of infarction in these 21 were disease progression in nonbypassed arteries or graft occlusion with arterial stenosis, or vice versa, and disease progression distal to a patent graft.(ABSTRACT TRUNCATED AT 250 WORDS)