During the 10-year period from 1979 to 1988 we evaluated 60 boys who were more than 9 years old and who had significant breast development (greater than 4 cm in diameter) around the time of puberty. An endocrine abnormality was identified in seven subjects. The pathology included Klinefelter's syndrome; 46,XX maleness; primary testicular failure; partial androgen insensitivity; fibrolamellar hepatocarcinoma; and increased aromatase activity. Eight of the remaining 53 subjects had underlying medical problems, five of them having neurologic disorders. The 45 remaining subjects were considered to have significant idiopathic gynecomastia, a condition sometimes referred to as macromastia. These boys tended to be both taller and heavier than average, the mean Z score for height being 1.4 SDs above the mean and the mean weight score being 2.7 SDs above the mean. This study underscores the observation that pathologic causes of marked pubertal gynecomastia are unusual. However, the potential for significant health problems among boys with marked breast development supports the need for an endocrine evaluation of all affected subjects. Our data also indicate that boys with marked idiopathic breast development have greater body mass than other boys of similar age. This may contribute in part to the greater breast development in these subjects.