The life expectancy of well controlled diabetics has been considered to be approximately that of the normal individual. A 20 year prospective study of the mortality of diabetics who applied for life insurance to the Equitable Life Assurance Society did not confirm this. The study population comprised 10,538 individuals with an average exposure of 7.9 years. During the period of the study 1,478 deaths were recorded, giving a mortality ratio of 335%. Mortality decreased with increasing age at diagnosis of diabetes. Mortality increased with increasing duration of the disease. Mortality was lowest in cases treated by diet only. Mortality in cases with poor control was two and one-half times that of cases with good control. Albuminuria on examination was an extremely unfavorable prognostic factor. Hypertension had a particularly adverse effect on the diabetic as compared to the non-diabetic, especially at ages under 40.