The role of androgens in the sexuality of men is still not completely clear. Men with severe hyperprolactinaemia frequently show mild hypogonadism, and many complain of loss of libido and penile erectile dysfunction (ED). We studied the night-sleep related erections and the penile response to visual erotic stimuli (VES) in 44 men: 13 with severe hypogonadism (Group 1; serum testosterone < 1.4 ng/ml), 10 with mild hypogonadism (Group 2; serum testosterone 2-3.5 ng/ml), nine with severe hyperprolactinaemia and mild hypogonadism (Group 3) and 12 control men (Group 4). All of the patients complained of loss of libido and ED. Group 1 showed significantly impaired night erections when compared with any of the other three groups, but no differences were detected between Groups 2, 3 and 4. The penile response to VES did not show any significant difference between the four groups, but was lower in Group 1 than in Group 4. These data confirm that night erections are androgen-dependent, but also suggest that there are two thresholds for serum testosterone: one below which sexual behaviour is impaired with normal night erections, and a lower threshold below which night erections are also impaired. The penile response to VES was confirmed as being only partially androgen-independent. Furthermore, hyperprolactinaemia does not affect night erections or the penile response to VES, suggesting that its effect on libido and sexual behaviour is due mainly to modulation of the psychological pattern of the patient.