The mortality rate from myocardial infarction is disproportionately high in diabetic patients. One explanation for this may be that diabetic patients incur more extensive myocardial necrosis. This possibility was examined in a three part study. Firstly, peak serum aspartate aminotransferase concentrations of all diabetic and non-diabetic patients admitted with myocardial infarction over a 16 year period were compared retrospectively. Secondly, peak aspartate aminotransferase concentrations in a series of diabetic patients and controls matched by age and sex were examined retrospectively. Thirdly, creatine kinase MB release and electrocardiographic measures of infarct size were investigated prospectively in a case/control study. Although cardiac failure and death were more common in the diabetic groups, there were no significant differences in estimates of infarct size between diabetic and non-diabetic patients in any of the studies. Therefore, the high case fatality rate amongst diabetic patients is not caused by increased myocardial damage. Presumably survival is prejudiced by factors operating before the infarction.