With age, some men develop symptoms resembling hypogonadism. Several cross-sectional and longitudinal studies have shown a decrease in testosterone levels with ageing in men. This finding has equally been observed in elderly men in good health. Testosterone levels decline progressively as of the thirties, at a rate which remains constant throughout life. While total testosterone levels decrease, sex hormone binding globulin (SHBG) levels on the contrary increase with age, with the result that the levels of free and non-SHBG-bound testosterone (corresponding to the fraction which is bioavailable to target cells) decrease more abruptly than that of total testosterone. Higher LH levels, decreased testosterone response to hCG and less Leydig cells all indicate that ageing induces partial testicular failure. However, the gonadotropic function is also affected in ageing. The hypothalamus-pituitary becomes more sensitive to gonad steroid feedback, LH pulse amplitude decreases, and the LH response to GnRH is blunted compared to the situation in young men. Thus LH level is not a valid index of androgen deficiency in elderly males. None of the androgen-dependent functions (libido, erection, sense of well-being, muscle mass and strength, fat mass, bone mass, erythropoiesis, etc.) are under exclusively androgen control, and there is no elderly male symptom which is completely specific to androgen deficiency. Thus, in elderly men, when clinical symptoms might indicate androgen deficiency, biological confirmation is needed. An assay which is independent of SHBG fluctuations is mandatory. Bioavailable testosterone assay by ammonium sulfate precipitation seems to us to be the optimum method for diagnosing androgen deficiency: it gives a reliable measurement for the testosterone fraction available to target cells, is adapted to clinical practice, and provides results that can be directly compared with current reference values for healthy young men.