RESUMOKlinefelter's syndrome (KS) results from a genetic deficiency (kariotype 47,XXY) that can lead to hypergonadotropic hypogonadism, azoospermia and underdevelopment of secondary sex characteristics. The exact mechanism that determines the androgenic deficiency is not yet understood, and the degree of Leydig cell dysfunction is variable. KS is a chronic disease with serious repercussions over the male reproductive system and also an important cause of infertility worldwide. We report on a 33 year-old male patient with progressive bilateral and painful gynecomastia, hypergonadotropic hypogonadism and associated emotional disturbances. The presence of signs and symptoms of androgen deficiency along with the demonstration of a 47,XXY kariotype led to the diagnosis of KS involving the patient in an uncommon syndrome of infertility and feminization and its biopsychosocial implications. A SÍNDROME DE KLINEFELTER (SK) é um estado intersexual, determinado geneticamente pela duplicação do cromossomo X, caracterizada por alterações tardias que se tornarão evidentes após a puberdade (1). É uma forma de manifestação de ginecomastia, estimando-se sua incidência em 1 caso para cada 850 homens (2). Consiste em uma das causas mais importantes de infertilidade masculina e hipogonadismo primário em nosso meio (3,4). Cerca de 10% dos azoospérmicos são portadores de SK (5).