We evaluated a family in which gynecomastia occurred in five males in two generations. In each affected subject, gynecomastia and male sexual maturation began at an early age. The ratio of the concentration of plasma estradiol-17 beta to that of plasma testosterone was elevated in each affected subject. In the three siblings with gynecomastia, the transfer constant of conversion of androstenedione to estrone (i.e., the fraction of plasma androstenedione that was converted to estrone as measured in the urine) was 10 times that of normal persons. The transfer constant of conversion of testosterone to estradiol-17 beta in the one subject studied also was 8-10 times that of normal men, whereas the transfer constants of conversion of estrone to estradiol-17 beta and of estradiol-17 beta to estrone were normal. Despite the elevation in extraglandular aromatase activity, there was a normal response of the hypothalamic-pituitary axis to provocative stimuli. This is the second documentation of gynecomastia that is associated with increased extraglandular aromatase activity, and the first time that the defect was found to be familial with a probable X-linked (or autosomal dominant, sex limited) mode of inheritance.