Age-adjusted 28-day case-fatality rates were higher among Mexican Americans than among non-Hispanic whites and higher among women than among men hospitalized for definite or possible myocardial infarction in Corpus Christi, Nueces County, Texas, from May 1, 1988, through April 30, 1990. The authors therefore examined whether these higher case-fatality rates were associated with greater prevalence of previously diagnosed coronary heart disease or diabetes; with greater age, frequency of definite myocardial infarction, or congestive heart failure; with higher values of indicators of severity of infarction, including peak creatine phosphokinase levels and scales prognostic of early mortality after myocardial infarction; and with differences in receipt of in-hospital therapy. The overall 28-day case-fatality rate among 1,228 patients hospitalized for myocardial infarction during a 24-month period was 7.3%. After adjustment for age; diabetes; myocardial infarction class (definite vs. possible); congestive heart failure; the Norris and Peel severity indices; peak total creatine phosphokinase; and receipt of thrombolytic therapy, aspirin, calcium channel blockers, beta-blockers, anticoagulants, angioplasty, and bypass surgery, the risk of 28-day case-fatality for Mexican Americans in relation to non-Hispanic whites was 1.49 (95% confidence interval 0.92-2.40). The corresponding risk for women in relation to men was 1.80 (95% confidence interval 1.12-2.89). These findings should alert clinicians to the high-risk status of these groups of patients.