The characteristics and clinical course of 1040 cases of acute myocardial infarction (AMI) among non-insulin-dependent diabetics (146) and nondiabetics (894) were compared. Patients with non-insulin-dependent diabetes mellitus (NIDDM) historically showed a greater percentage of AMI, angina, and risk factors than nondiabetic patients. Although the degree of left-ventricular function upon admission (according to the Killip and Kimball scores) was similar in both the diabetic and nondiabetic groups, the prevalence of hypertension and hypercholesterolemia was significantly higher in the NIDDM patients. All told, NIDDM cases were 1.73 [relative risk (RR)] times more likely to die of AMI than nondiabetic patients. The age factor and the presence of shock of any type also significantly increased the case-fatality rate. Diabetic patients showed signs of successful reperfusion less often than control subjects, an event that was closely associated with their case-fatality rate. In the NIDDM group, both the age and gender factor as well as a history of either casual or in-hospital clinical events such as cardiogenic shock, reinfarction, unsuccessful reperfusion, and incidence of anterior AMI along with either pain or previous angina were clear prognosticators of poor outcome from AMI. In the nondiabetic group, cardiogenic shock and hypertension were indicators of poor prognosis. These results would suggest that an improvement in the incidence of successful reperfusion in NIDDM patients, particularly in the face of clinical indicators of poor AMI prognosis, could decrease the high AMI mortality currently observed in these patients.