Three hundred and seventy diabetics and an equal number of matched controls were followed up over a ten-year period to compare the mortality of diabetics with that of nondiabetics, and to determine how the longevity of diabetics is affected by various complicating conditions and by the severity of the diabetes. During the ten-year study period, the death rate of the diabetics was 25.4% compared to 9.7% among the controls, a ratio of about 2.6. Some of the excess mortality was accounted for by (1) a higher prevalence of hypertension, overweight, coronary heart disease, and kidney disease, and (2) the severity of the diabetes, as indicated by a high insulin dosage, early onset, and recurrent glycosuria. However, the mortality of those with mild diabetes and those without complicating diseases was also substantially greater than that of the nondiabetics.