Three hundred and fifty-one patients with histologically proven gynecomastia were studied. It was observed that pubertal and hormone-induced gynecomastias tend to be bilateral and diffuse, whereas idiopathic and nonhormonal drug-induced gynecomastias are usually unilateral and discrete. It was also observed that gynecomastia of short duration is of the florid type, whereas gynecomastia of long duration is usually of the fibrous type. These findings support the conclusion that etiologic factors have a fundamental influence on the gross appearance and growth pattern of gynecomastia. The histologic appearance is related to the duration of gynecomastia, irrespective of etiologic factor. It is also postulated that the hyperplastic breast tissue undergoes a process of maturation or senescence, proceeding from the florid to the fibrous type.