OBJECTIVE--To document the relative importance of endogenous sex steroids in modulating the frequency of orgasms, the dominant aspect of sexual behaviour in healthy eugonadal men. DESIGN--Measurement of adrenal and testicular sex steroids in a sample of army recruits and study of their relation to frequency of orgasms ascertained by questionnaire after potential confounding variables were controlled for. SETTING--Military campus and military hospital laboratories in Athens, Greece. SUBJECTS--92 consecutively enrolled healthy male recruits aged 18-22 years. MAIN OUTCOME MEASURES--Weekly number of orgasms. Serum concentrations of testosterone, dehydroepiandrosterone sulphate, dihydrotestosterone, oestradiol, oestrone, delta-4-androstenedione, and sex hormone binding globulin. RESULTS--Serum dihydrotestosterone concentration was the only independent hormonal predictor of the frequency of orgasms; an increase in concentration of 1.36 nmol/l (about 2 SD) corresponded to an average increase of one orgasm a week. CONCLUSIONS--Differences in concentrations of circulating dihydrotestosterone within the normal range may represent a major predictor of sexual activity in healthy young men.