Forty years after the introduction of estrogens, the debate over the risks and benefits of estrogen replacement therapy in postmenopausal women is still not entirely settled. In contrast, the issues of testosterone replacement in older men are just beginning to be addressed. There is agreement that total, free, and bioavailable testosterone (ie, not bound by sex hormone-binding globulin) levels decline progressively with advancing age (Harman and Tsitouras, 1980, 1982; Murono et al, 1982; Nieschlag et al, 1982; Zumoff et al, 1982; Bremner and Prinz, 1983; Davidson et al, 1983; Nankin and Calkins, 1986; Tenover et al, 1987; Gray et al, 1991; Vermeulen, 1991; Simon et al, 1992; Rudman and Shetty, 1994; Morley et al, 1997; Zmuda et al, 1997; Harmon et al, in press), and that many of the physiological changes that occur with advancing age such as loss of bone and muscle mass; increased fat mass; impairment of physical, sexual, and cognitive functions; loss of body hair; and decreased hemoglobin levels, are similar to those associated with androgen deficiency in young men. However, the beneficial effects of testosterone replacement on health-related outcomes in older men with low testosterone levels have yet to be demonstrated, and the risks of long-term testosterone administration, particularly the risks of prostate cancer and heart disease, remain unknown. Therefore, it is premature to make a general recommendation about testosterone supplementation in older men. Despite the paucity of efficacy and safety data, the sales of testosterone and other androgenic products have witnessed explosive growth recently