Ninety-two cases of diabetic ketoacidosis were identified in residents of Rochester, Minnesota, over a 52-year period; 42% occurred in patients with juvenile-onset diabetes. The most common cause of diabetic ketoacidosis was infection. In 23% of the patients, diabetic ketoacidosis was the initial manifestation of diabetes mellitus. The frequency of retinopathy, neuropathy, and glomerulosclerosis was significantly increased in the surviving diabetic patients with ketoacidosis in comparison with diabetic patients matched for age and sex who did not experience ketoacidosis. Seven deaths occurred in the first 24 hours of diagnosis of ketoacidosis, but only one could be attributed to ketoacidosis alone. An additional six patients died within 48 hours of the recognition of ketoacidosis. The major cause of all deaths occurring within 48 hours of hospitalization for ketoacidosis was myocardial infarction.