A family has been presented in which gynecomastia is present in 4 males. A study was possible of 3 of the 4 family members. Normal values were obtained for urinary strogens, 17-ketosteroids, 17-hydroxycorticosteroids, pregnanediol and pregnanetriol. One patient had evidence of hypothyroidism which did not appear to contribute to the gynecomastia. In the absence of endocrinologic abnormalities or contributing systemic disease, an inherited heightened tissue sensitivity to a normal endocrine environment is postulated to explain this familial phenomenon.