The role of estrogens in male physiology has become more evident, as a consequence of the discovery of human models of estrogen deficiency such as estrogen resistance or aromatase deficiency. In males, testosterone is the major source of plasma estradiol, the main biologically active estrogen, only 20% of which is secreted by the testes. Plasma estrone, 5% of which is converted to plasma estradiol, originates from tissue aromatization of, mainly adrenal, androstenedione. The plasma concentration of estradiol in males is 2-3 ng/dl and its production rate in blood is 25-40 micrograms/24 h; both of these values are significantly higher than in postmenopausal women. Plasma levels of estradiol do not necessarily reflect tissue-level activity as peripherally formed estradiol is partially metabolized in situ; thus, not all enters the general circulation, with a fraction remaining only locally active. Of the factors influencing plasma estradiol levels, plasma testosterone is a major determinant. However, the age-associated decrease in testosterone levels is scarcely reflected in plasma estradiol levels, as a result of increasing aromatase activity with age and the age-associated increase in fat mass. Free and bioavailable estradiol levels do decrease modestly with age as does the ratio of free testosterone to free estradiol, the latter testifying to the age-associated increased aromatization of testosterone. Estradiol levels are highly significantly positively related to body fat mass and more specifically to subcutaneous abdominal fat , but not to visceral (omental) fat. Indeed, aromatase activity in omental fat is only one-tenth of the activity in gluteal fat. Estrogens in males play an important role in the regulation of the gonadotropin feedback, several brain functions, bone maturation, regulation of bone resorption and in lipid metabolism. Moreover, they affect skin metabolism and are an important factor determining sex interest in man.