The second objective of this study is to determine whether physiologic testosterone replacement of older men with low testosterone levels improves health-related quality of life. Aging-associated decline in physical, sexual, and cognitive functions contributes to diminished quality of life in older men. Although the pathophysiology of impairment in each of these subdomains of health-related quality of life is complex and multifactorial, one correctable cause of the diminished quality of life in older men is the decrease in serum testosterone concentrations. Total and free testosterone (T) levels decline with advancing age in normal men, with a significant number of men meeting usual criteria for hypogonadism by the sixth to seventh decades. Spontaneous and experimentally-induced androgen-deficiency in young men is associated with decreased muscle mass and strength and impaired sexual function. Because loss of muscle mass and function contributes to diminished health-related quality of life (HRQOL) in older men, anabolic therapies such as testosterone that increase muscle mass and strength, would be expected to improve physical function. In older men with low testosterone levels, testosterone might also improve sexual function and marital interaction. A growing body of literature suggests that testosterone impacts neuronal functioning and may affect cognitive performance. Because physical, sexual, and cognitive functions are important determinants of health-related quality of life, testosterone replacement of older men with low testosterone levels would be expected to improve general health perceptions.