The age-associated, progressive changes in body composition are paralleled by a progressive decrease in hormone levels, among which there are testosterone (T) and growth hormone (GH). Moreover, the age-associated changes are rather similar to the changes in body composition seen in hypogonadism or hyposomatotropism, respectively. These age-associated changes in body composition are rather important: a decline in muscle mass by 20%-40% between the ages of 25 and 75, a doubling of fat mass and a decrease in bone mineral density by 0.3% per year after age 35. They have important functional and metabolic consequences. Most studies show a significant positive correlation between muscle mass and bone mineral density, respectively, and plasma T or GH levels, whereas abdominal fat mass is generally negatively correlated with T and GH levels. Whether, however, the hormone levels play a causal role or are the consequence of the changes in body composition is still an unsettled problem. The beneficial effects of hormone substitution on body composition suggest, nevertheless, that T and GH are at least co-determinants of the observed age-associated changes.