High-dose methadone is well known to cause testosterone deficiency and sexual dysfunction in opioid-dependent men. Buprenorphine is a new drug for the pharmacotherapy of opioid dependence. Its influence on the gonadal axis has not been investigated to date. We therefore assayed testosterone, free testosterone, estradiol, SHBG, LH, FSH, and prolactin in 17 men treated with buprenorphine. Thirty-seven men treated with high-dose methadone and 51 healthy blood donors served as controls. Sexual function and depression were assessed using a self-rating sexual function questionnaire and the Beck Depression Inventory. Patients treated with buprenorphine had a significantly higher testosterone level [5.1 +/- 1.2 ng/ml (17.7 +/- 4.2 nmol/liter) vs. 2.8 +/- 1.2 ng/ml (9.7 +/- 4.2 nmol/liter); P < 0.0001] and a significantly lower frequency of sexual dysfunction (P < 0.0001) compared with patients treated with methadone. The testosterone level of buprenorphine-treated patients did not differ from that of healthy controls. In conclusion, we demonstrated for the first time that buprenorphine, in contrast with high-dose methadone, seems not to suppress plasma testosterone in heroin-addicted men. To this effect, buprenorphine was less frequently related to sexual side effects. Buprenorphine might therefore be favored in the treatment of opioid dependence to prevent patients from the clinical consequences of methadone-induced hypogonadism.