OBJECTIVE: We have previously shown that hypogonadotrophic hypogonadism is common in middle-aged patients with type 2, but not with type 1, diabetes. We have now investigated the total and free testosterone concentrations in young (aged 18-35 years) type 1 and type 2 diabetic patients. RESEARCH DESIGN AND METHODS: In this study carried out in a tertiary referral center, serum concentrations of total and free testosterone were measured in 38 type 1 diabetic (mean age 26.45 +/- 0.89 years) and 24 type 2 diabetic (mean age 27.87 +/- 0.97 years) subjects. The mean BMI of type 1 and type 2 diabetic patients was 27.41 +/- 1.18 and 38.55 +/- 2.04 kg/m(2), respectively (P < 0.001). RESULTS: The mean total testosterone concentration of type 1 and type 2 diabetic patients was 22.89 +/- 1.23 and 11.14 +/- 0.99 nmol/l, respectively (P < 0.001). The mean free testosterone concentration of type 1 and type 2 diabetic patients was 0.489 +/- 0.030 and 0.296 +/- 0.022 nmol/l, respectively (P < 0.001). Eight of 24 (33%) type 2 diabetic patients had subnormal free testosterone concentrations (<0.225 nmol/l). Using an age-based reference range, 14 of 24 (58%) type 2 diabetic patients had low free testosterone concentrations (<0.278 nmol/l). Three of 38 (8%) type 1 diabetic patients had free testosterone concentrations below the lower limit of normal (P = 0.02 when compared with type 2 diabetes). Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations in type 2 diabetic patients with low free testosterone concentrations were in the normal range and were similar to those in type 1 diabetic patients. CONCLUSIONS: Young type 2 diabetic patients have significantly lower plasma concentrations of total and free testosterone and inappropriately low LH and FSH concentrations with a very high prevalence of hypogonadotrophic hypogonadism, when compared with type 1 diabetic patients of a comparable age. The potential implications for their sexual and reproductive function during prime reproductive years are profound.