The effect of diabetes on survival after myocardial infarction (MI) was examined in a prospective population-based study of individuals hospitalized with MI in a bi-ethnic community of Mexican-Americans and non-Hispanic whites. Among Mexican-Americans, 54% (331 of 610) had diabetes compared with 33% (192 of 589) of non-Hispanic whites (P < 0.001). Among those with diabetes, the prevalence of a history of a cardiac event before the index admission was significantly higher (odds ratio = 1.4, 95% confidence interval CI 1.1-1.8) than among nondiabetic subjects. During the index hospitalization, diabetic subjects received cardiac catheterization less frequently than did nondiabetic subjects (45.1 vs. 51.5%, P = 0.03). Diabetic subjects had lower estimated ejection fractions, and the number of coronary arteries with significant obstruction (> 75%) was higher (P < 0.001). The peak creatine phosphokinase and creatine phosphokinase myocardial isoenzyme (CK-MB) levels were similar in diabetic and nondiabetic subjects. Despite a similar infarct size, diabetic subjects had a higher incidence of congestive heart failure (relative ratio = 2.2, 95% CI 1.7-2.8), more adverse indexes of short-term and long-term prognosis, and a longer average hospital stay (12.1 vs. 8.9 days, P < 0.01). After adjustment for age, sex, and ethnicity, the cumulative risk for total mortality, over 44 months of follow-up, was 37.4% among diabetic compared with 23.3% among nondiabetic subjects (P < 0.001). Diabetic subjects had a higher 28-day case-fatality rate post-MI as well as higher long-term mortality.