Male breast disease, although overshadowed by its female counterpart, is still a problem which often comes to the attention of the radiologist. The major importance of mammography is differentiating unilateral gynecomastia from cancer. We now recognize four xeroradiographic patterns of gynecomastia: (1) increased ducts only; (2) ductal hyperplasia mimicking adenosis; (3) small ducts with stromal proliferation; and (4) fatty replacement only. These reflect the duration of the process. Gynecomastic masses are central, smooth, and most important, extend from the nipple outward, usually bisecting the midplane. Ducts may be visible. Carcinoma is usually central, dense, with irregular spiculated margins and, unlike gynecomastia is rarely in the midplane of the nipple, although it can be retroareolar in location. There may be concomitant skin changes or lymphadenopathy seen on the xerogram and these never occur in gynecomastia. There are numerous etiologies for gynecomastia, and these have been discussed in detail. It is thought that the radiologist should be aware of the pathophysiology of male breast disease and understand its significance.