A group of 205 patients hospitalized with myocardial infarction 2 to 162 months (mean 66) after bypass surgery and 205 control patients with myocardial infarction were compared and followed up for 34 +/- 25 months after hospital discharge. At baseline the postbypass group contained more men (p less than 0.03) and more patients with previous myocardial infarction (p less than 0.06), but the groups were otherwise comparable. Indexes of infarct size were lower in postbypass patients: sum of ST elevation, QRS score, peak serum creatine kinase (CK) (1,115 +/- 994 versus 1,780 +/- 1,647 IU/liter) and peak MB CK (all p less than or equal to 0.001). Postmyocardial infarction ejection fraction was 45 +/- 15% in the postbypass group and 43 +/- 15% in the control group (p = NS); in-hospital mortality rate was 4 and 5%, respectively (p = NS). When patent grafts were taken into account, the two groups were comparable in extent of coronary artery disease. At 5 years after discharge, cumulative mortality was similar in the postbypass and control groups (30 versus 25%, respectively, p = NS). However, postbypass patients had more reinfarctions (40 versus 23%, p = 0.007), more admissions for unstable angina (23 versus 18%, p = 0.04) and more revascularization procedures (34 versus 20%, p = 0.04) than did control patients. The total for these events at 5 years was 70% in the postbypass group and 49% in the control group (p = 0.001). Thus, although patients with previous bypass surgery who develop acute myocardial infarction have a smaller infarct, their subsequent survival is no better than that of other patients with acute myocardial infarction. They experience more reinfarctions and unstable angina. Previous bypass surgery is an important clinical marker for recurrent cardiac events after myocardial infarction.