In a Southern California population-based cohort of 985 men aged 40-79 years in 1972-1974, 110 men with diabetes had significantly lower mean levels of endogenous plasma testosterone and sex hormone-binding globulin than did 875 nondiabetic men. Mean androstenedione, estrone, and estradiol levels did not differ significantly by diabetic status. The testosterone and sex hormone-binding globulin differences were reduced but not eliminated after adjustment for age and body mass index (weight (kg)/height (m)2). Twenty-one percent of diabetic men compared with 13% of nondiabetic men had testosterone levels below a categorically defined normal level of 3,500 pg/ml. Testosterone was inversely related to degree of glycemia in the whole cohort. Throughout the whole range of plasma glucose, there was a stepwise decrease in mean testosterone levels per categorical increase in fasting plasma glucose, apparent in both diabetics and nondiabetics. The clinical and physiologic implications of the reduced testosterone levels in diabetic men merit further investigation.