Background: Gynecomastia can be physiological or pathological. A limited study of gynecomastia is recommended during puberty and in the elderly, ages in which gynecomastia is usually considered physiological. Other authors suggest that this condition should be studied when it is painful, rapidly growing, of recent onset, when its diameter is more than 4 cm and when is associated to testicular masses. Aim: To investigate the causes of gynecomastia and to evaluate the above mentioned criteria to exclude pathological conditions. Material and methods: Prospective study of 177 patients aged 20 to 83 yeas consulting for gynecomastia. All were subjected to a standardized study including a clinical examination and measurement of plasma estradiol and testosterone levels. Results: Pony one percent of gynecomastias were considered pathological and the rest, physiological. Among pathological conditions, 18 patients had an endocrine etiology (hypogonadism in ten patients, estrogen secreting tumors in three, hyperestrogenism of unknown etiology in, four and peripheral resistance to androgens in one), in 2 7 it was secondary to medications and in 13 it was secondary to other causes (idiopathic, pesticide exposure, alcoholism, diabetes or re feeding). In 79% of 86 patients of less than 20 years the condition was physiological and in four of five elderly subjects, it was pathological. Thirty nine percent of pathological gynecomastias lacked the signs and symptoms that according to authors should prompt a thorough study. Conclusions: All patients with gynecomastia should be studied with a complete medical history and the measurement of estradiol and testosterone levels. The criteria proposed to conduct minimal studies in gynecomastia, would miss a large volume of pathological conditions.