Cognitive function is often impaired in type I diabetes mellitus, especially when onset is in early childhood. Limited evidence suggests similar impairment in the much larger population of older persons with type II diabetes. We report here the results of 13 measures of mental efficiency in persons between 65 and 77 years of age without gross mental impairment. Nineteen persons with type II diabetes were compared with 19 controls with normal glycosylated hemoglobin values and to seven persons not previously known to have diabetes but who had elevated glycosylated hemoglobin. Significant differences (P less than .05) between the nondiabetic control and diabetic groups were noted on eight of the 13 tests. The group of subjects with elevated glycosylated hemoglobins showed levels of cognitive performance intermediate between the normal and known diabetic groups on the majority of tests. These findings confirm that mental efficiency is frequently impaired in older persons with diabetes. Given the high prevalence of diabetes in the aging population we suggest that diabetes may be another important factor contributing to premature loss of cognitive abilities in older people.