Aging of humans is associated with a loss of muscle mass and function, and an increase in fat mass. Epidemiologic studies have demonstrated a correlation between bioavailable testosterone concentrations and fat-free mass and muscle strength. Testosterone replacement in older men with low testosterone levels increases fat-free mass and muscle strength, and decreases fat mass. However, we do not know whether testosterone replacement improves physical function and other health-related outcomes, or reduces the risk of disability, falls, or fractures in older men with low testosterone levels. The long-term risks and benefits of testosterone supplementation in older men are not known.