The role of testosterone on sexual desire is well established. However, the effects of low testosterone levels in the pathophysiology of erectile mechanism in humans remains unclear. Recent evidence indicate that approximately 10-20% of men with erectile dysfunction (ED) have hormonal abnormalities, raising up to 35% over the age of sixty. It is now clear that sexual desire and erectile function in humans are both responsive to androgens, probably at different threshold values. In fact, different degrees of testosterone deficiency may determine a sequence of molecular penile events leading to reduced capacity of penile smooth muscle and endothelial cells of relaxation, without greatly affecting sexual desire. Also, androgens may directly control the expression and activity of phosphodiesterase type-5 in human corpus cavernosum. In some selected men with total testosterone below 10-13nmol/l and/or free testosterone below 200-250pmol/l, androgen supplementation may improve therapeutic efficacy of phosphodiesterase type-5 inhibitors. For ageing men with partial androgen deficiency (PADAM) who fail first-line oral treatments in whom androgens are not contraindicated, a combination of testosterone and phosphodiesterase type-5 inhibitors may be considered to improve erectile function and improve the quality of life.