Although, in distinction to middle aged women, in middle aged men there does not occur a sudden arrest of gonadal functions, fertility persisting until very old age, aging in men is, nevertheless, associated with an gradual decline of both endo- and exocrine testicular function. Whereas age has in fact only minimal effects on the quality of the ejaculate, endocrine function declines steadily with age and at age 75 years, mean plasma testosterone levels are only 65% of levels in young adults whereas over 25%, of these men have bioavailable testosterone levels below the lower normal limit in young adults. The interindividual variations in the plasma levels are, however, very important and a quarter of men over 75 years old, have still testosterone levels within the upper quartile of values in young men. Aging is accompanied by a series of signs and symptoms, many of which are rather similar to those observed in young hypogonadal males. The etiology of these signs and symptoms is often multifactorial, and very few correlations have been found between symptoms and plasma testosterone levels. Nevertheless, there is good evidence that the age associated decrease in testosterone levels is at least a co-determinant of these symptoms and testosterone supplementation has shown favorable effects on many of them. Side effects of this substitutive therapy are minimal when care is taken to keep plasma testosterone levels within the physiological range. Clinical prostatic carcinoma is an absolute contra-indication for testosterone supplementation. So far, there are no indications that testosterone would stimulate the evolution of as subclinical prostatic carcinoma to a clinical carcinoma but it should be recalled that so far, only a small number of elderly males received substitutive androgen treatment for longer periods in controlled studies. Hence, although side effects are generally minimal, one should, nevertheless, await the results of larger, long term, well-controlled studies before to recommend the routine testosterone substitution of elderly men.