Fasting blood glucose (FBG) level and oral glucose tolerance (OGT) were determined in 169 patients within 72 hours of an acute myocardial infarction. Elevated FBG levels were found in 47.5% and a reduced OGT in 72.5%. Of 32 patients who died in the hospital, FBG value was elevated in 72% and the OGT was abnormal in 89%. Of 91 patients who survived longer than six years, the initial FBG level had been elevated in 33%, and the OGT had been abnormal in 67%. Eighty percent of the group with initially raised FBG values had either latent or overt diabetes, while more than 95% of the patients with initially normal FBG values had a normal OGT. Fifty-five percent of the patients with abnormal OGT during myocardial infarction showed normal OGT six years later. The FBG level shortly after an acute myocardial infarction is a better guide to prognosis and to the prediction of subsequent development of diabetes mellitus than the OGT test.