In 5 patients (2 women and 3 men, aged 16-36 years), diabetic ketoacidosis developed without precipitating illness. Pancreatic islet cell antibody was negative, and the duration of insulin dependency was shorter than 4 weeks. Hemoglobin A1c was < or = 6.3% for the mean period of 2.8 years thereafter, with diet therapy alone in 4 and with 5 mg glyburide in 1. Four were overweight before the development of diabetes, and 3 of them positive for family history of adult-onset, non-ketotic diabetes. Frequency of human leukocyte antigen B61 was increased significantly in the patients. In a patient not previously overweight, family history of diabetes was negative, and human leukocyte antigen haplotypes common in insulin-dependent diabetes mellitus were accumulated. Serum immunoreactive insulin was within normal range or supranormal with normal glucose tolerance after recovery. The patients closely resemble black Americans with ketoacidosis-onset non-insulin dependent diabetes.