Erectile function progressively declines with age, although most old men's erectile rigidity remains adequate for vaginal intercourses. Recently it has been demonstrated that a significant proportion of men over 60 years of age has a biochemical hypogonadism (defined as below the 2.5th percentile for young men). It is generally accepted that erectile response in mammals is regulated by androgens both centrally and peripherically, but in male ageing a casual relationship between androgen levels and erectile function has not been established yet. The present study was designed to evaluate the possible relationships between circulating androgens and sexual activity in a group of healthy old men without erectile dysfunction. Forty men with a mean age of 68 +/- 6 years (range 61-75 years) were recruited among healthy subjects undergoing periodic health screening. The control group consisted of 30 young volunteers with a mean age of 33 +/- 5 years (range 26-40 years). These subjects had good general health and stable relationship with at least one successful intercourse for week. All subjects performed: 1) hormonal assessment (total testosterone, free testosterone, DHT, FSH and LH); 2) nocturnal penile tumescence monitoring during two consecutive nights. The findings of this study show a significant decrease in frequency, duration and degree of nocturnal erectile episodes in old men with respect to those observed in young controls. The alteration of these parameters was more evident in old subject showing a reduction of testosterone plasma levels. In hypotestosteronemic subjects DHT treatment significantly improved nocturnal penile tumescence parameters even if these still differed from those of controls (p<0.001). On the basis of these observations we hypothesize that hypoandrogenism in old men, also in presence of normal sexual activity, may influence locally the erectile mechanisms. However hypoandrogenism, associated to other conditions negatively influencing the erectile activity, may be an important co-factor in the induction of erectile dysfunction in aging.