Testosterone is the main male hormone and the testicle is the main gland where it is produced during the whole life. Hormone production falls progressively. There is a clear relation between the hormone and sexual behaviour. Hormone deficit causes alterations in different organs and systems. Low testosterone circulating values are related to many conditions as welt as to the increase of mortality and morbidity. During the last years, treatment with testosterone to avoid or improve sexual disorders and other conditions has been promoted. Testosterone plasma level under which men should be diagnosed and treated is still in dispute. The communication of side effects of the treatment has attracted health authorities' attention in different countries and produced diverse reactions over the international scientific community. Late-onset hypogonadism syndrome must be diagnosed carefully and never under just isolated hormonal analysis. Low testosterone values get very often corrected without needing hormonal substitution, but just by treating the causes which give rise to it. (C) 2014 Asociacion Espanola de Andrologia, Medicina Sexual y Reproductiva. Published by Elsevier Espana, S.L.U. All rights reserved.