To measure the comparative prevalence of testicular involvement in borderline lepromatous (BL) and lepromatous (LL) leprosy patients, serum FSH, LH, and total testosterone levels were measured in 42 LL and 21 BL subjects. Serum FSH levels were elevated in 19% of BL and in 86% of LL patients. Serum LH values were increased in 10% of BL and in 79% of LL patients. Total serum testosterone values below the normal limit of 280 ng/dl were not found in BL subjects but were present in 31% (13) of the LL cases. By measuring serum free testosterone in patients with low-normal total values, one BL and an additional five LL patients could be identified as below normal limits, i.e., less than 50 pg/ml. Thus, androgen deficiency was present in 5% of BL and in 43% of LL subjects. All of these differences between the BL and LL patients were statistically significant.