Testosterone deficiency accounted for decreases in lean mass, muscle size, and strength. Estrogen deficiency primarily accounted for increases in body fat; and both contributed to the decline in sexual function. As serum testosterone levels decline, there is a concomitant decline in serum estradiol levels. And Estradiol deficiency may be important in the pathogenesis of some consequences of male hypogonadism. Visceral Fat increases when estrogen is blocked in Testosterone administration. Body fat percentage increases with blocking of Estradiol. Because increases in visceral fat reduce insulin sensitivity and are associated with diabetes and the metabolic syndrome, the marked increase in intra-abdominal fat with aromatase inhibition could portend an increase in cardiovascular disease with long-term estrogen deficiency. The findings in this article show that estrogen has a fundamental role in the regulation of body fat and sexual function, coupled with evidence from prior studies of the crucial role of estrogen in bone metabolism, indicates that estrogen deficiency is largely responsible for some of the key consequences of male hypogonadism.