In order to study the effects of sildenafil on human sleep-related erections according to the state of androgenization, we evaluated the effects of sildenafil on sleep-related erections in hypogonadal men before and during testosterone replacement treatment and in control subjects. Twenty-four hypogonadal men and twenty-four healthy men as a control group were enrolled. All hypogonadal subjects had very low testosterone levels: <200 ng/dl (8.93 nmol/L). All subjects underwent nocturnal penile tumescence and rigidity monitoring (NPTRM) for 3 consecutive nights and randomly assumed sildenafil 50 mg or placebo 1 hour before bedtime in the second or in the third night of nocturnal penile monitoring. The hypogonadal subjects were tested twice, without replacement treatment (H-T) and after at least 6 months of testosterone replacement therapy (H+T). The subjects of the control group (C) were tested once. The following parameters of sleep-related erections were analyzed: total number of valid erections, total duration of both rigidity > 70% and increase in penile circumference > 30 mm, maximum rigidity, and maximum increase in penile circumference. NPTRM parameters were reduced in hypogonadal men before testosterone treatment (H-T+P) when compared with control subjects taking placebo (C+P). NPTRM parameters after testosterone (H+T+P) and sildenafil (H-T+S) administration were similar to that of control subjects taking placebo (C+P). When the statistical analysis was restricted to the hypogonadal men before testosterone treatment, sildenafil alone significantly increased NPTRM parameters when compared with placebo (H-T+S vs H-T+P). Testosterone restored normal erections when administered to hypogonadal subjects (H+T+P vs H-T+P), in hypogonadal men, however, the combined treatment (sildenafil plus testosterone) resulted in the maximum positive effect on NPTRM parameters. When the increase from baseline was analyzed, the effects of testosterone plus sildenafil resulted higher than the sum of the effects of both drugs used alone. In conclusion, sildenafil administered at bedtime improves sleep-related erections in hypogonadal men, suggesting that the nitric oxide pathway may be pharmacologically enrolled and enhanced despite low serum testosterone. Furthermore, these data strongly support the idea of a synergic effect on sleep-related erections of sildenafil and testosterone.