PIP: Literature on the role of estrogens in men is reviewed. The primary active estrogens in males and females are estradiol-17beta (E2), estrone (E1), and estriol (E3). The active constituents of serum E2 and testosterone (T) are those which are not bound by testosterone-estrogen binding globulin (TEBG). The concentration of TEBG is stimulated by estrogen and suppressed by androgens. Both E1 and E2 appear to be derived from the peripheral metabolism of T and androstenedione. The metabolism and physiological roles of estrogens in men are briefly discussed. The association of gynecomastia with puberty, cirrhosis of the liver, hyperthyroidism, chronic renal failure, refeeding gynecomastia, administration of digitalis and diuretics, neoplasms, and hypergonadotropic hypogonadism is reviewed. In older men, the ratio of free E2:T is increased. The relationship of andorgens, estrogens, and male sex behavior is briefly reviewed. Areas for future research include the mechanisms by which estrogens and androgens exert antagonistic effects on similar tissues, variations in the fractional conversionr ate of androgens to estrogens, the etiology of pubertal gynecomastia, the role of the free E2:T ratio in male social and sexual behavior, and the interrelation between behavior, nutrition, hormone secretion, and degenerative changes such as benign prostatic hyperplasia.