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To review the role and significance of hypogonadism, defined as a low testosterone (T) level, in erectile dysfunction (ED). Review of literature. Serum T is below 3 ng/ml in 12% of ED patients, including 4% before and 15% after the age of 50. Replacement studies in men with severe hypogonadism demonstrate that sexual desire and arousal, as well as the frequency of sexual activity and spontaneous erections are clearly T-dependant. Psychic erections are partly T-dependant. The effects of T upon sexual function are dose-dependant up to a threshold level that is consistent within an individual, but markedly variable between individuals, ranging from 2 to 4.5 ng/ml. More evidence is required to confirm a significant impact of T on the intrapenile vascular mechanisms of erections in men as it is the case in animals. No convincing association of T with ED has been found in epidemiological studies. As concerns clinical experience, although a meta-analysis of the randomized controlled trials established that T therapy consistently restores erectile function in young hypogonadal patients with T below 3.46 ng/ml, the effects of this treatment have been mostly disappointing when used alone in older patients consulting for ED who are subsequently diagnosed to have hypogonadism following routine T measurement. These poor results may probably be explained by the high prevalence of co-morbidities, and by the fact that ED itself may induce hypogonadism. Combination therapy with T and PDE5 inhibitor (PDE5I) may be effective in the hypogonadal ED patients when T therapy alone fails. However, more evidence is required to confirm the hypothesis that a minimum level of T is required for a complete effect of PDE5I in certain men, since a PDE5I was able to restore complete erections in severely hypogonadal men. Though a low T level is not always the only cause of ED in hypogonadal ED patients, there are important benefits in screening for hypogonadism in ED. A low T level justifies a 3 month trial of T therapy, before combining a PDE5I if T therapy alone fails [ABSTRACT FROM AUTHOR]