In the present study, we investigated the effects of a steroid 5 alpha-reductase inhibitor, finasteride, when given orally (5 mg/day), on the serum levels of gonadal, hypophyseal, and adrenal hormones and the clinical significance of these effects. Forty-eight patients with a mean age of 63 (range 49-81) were included in the study. All patients had symptoms of benign prostatic hyperplasia. Serum levels of testosterone, dihydrotestosterone, follicle-stimulating hormone (FSH) luteinizing hormone (LH), prolactin, aldosterone, cortisol, and dehydroepiandrosterone were determined before the study. The degree of symptoms in each patient and serum prostate specific antigen levels were determined together with uroflowmetric studies. Sexual status of the patients was also assessed with a self-administered questionnaire. All patients received finasteride, 5 mg/day, for 6 weeks. All of the above mentioned studies were repeated at month 3 and month 6. All of the patients had baseline hormonal values within the normal range. At month 3, the dihydrotestosterone level decreased by 60%, while the testosterone level increased by 15%. FSH and LH levels decreased by 24% and 16%, respectively. The changes in the serum levels of these hormones were further evident at month 6. No significant changes were noted in the serum levels of prolactin, aldosterone, cortisol, and dehydroepiandrosterone. Thirty-six patients (75%) were judged to be potent before the treatment. Finasteride caused erectile dysfunction in 8 patients (22%) by month 3 and in 12 (33%) by month 6. A substantial improvement was noted in symptoms of benign prostatic hyperplasia in all patients. The serum prostate specific antigen level decreased by 42% and 50% at month 3 and at month 6, respectively. Continued administration of finasteride, 5 mg/day alters the serum levels of testosterone, dihydrotestosterone, FSH, and LH significantly. Finasteride also causes sexual dysfunction in a substantial number of patients and should be offered with caution to patients who have an active sexual life.