Gynecomastia: Enlargement of the BREAST in the males, caused by an excess of ESTROGENS. Physiological gynecomastia is normally observed in NEWBORNS; ADOLESCENT; and AGING males.Breast Neoplasms, Male: Any neoplasms of the male breast. These occur infrequently in males in developed countries, the incidence being about 1% of that in females.Klinefelter Syndrome: A form of male HYPOGONADISM, characterized by the presence of an extra X CHROMOSOME, small TESTES, seminiferous tubule dysgenesis, elevated levels of GONADOTROPINS, low serum TESTOSTERONE, underdeveloped secondary sex characteristics, and male infertility (INFERTILITY, MALE). Patients tend to have long legs and a slim, tall stature. GYNECOMASTIA is present in many of the patients. The classic form has the karyotype 47,XXY. Several karyotype variants include 48,XXYY; 48,XXXY; 49,XXXXY, and mosaic patterns ( 46,XY/47,XXY; 47,XXY/48,XXXY, etc.).Hypogonadism: Condition resulting from deficient gonadal functions, such as GAMETOGENESIS and the production of GONADAL STEROID HORMONES. It is characterized by delay in GROWTH, germ cell maturation, and development of secondary sex characteristics. Hypogonadism can be due to a deficiency of GONADOTROPINS (hypogonadotropic hypogonadism) or due to primary gonadal failure (hypergonadotropic hypogonadism).Feminization: Development of female secondary SEX CHARACTERISTICS in the MALE. It is due to the effects of estrogenic metabolites of precursors from endogenous or exogenous sources, such as ADRENAL GLANDS or therapeutic drugs.Breast Diseases: Pathological processes of the BREAST.Angiomatosis: A condition with multiple tumor-like lesions caused either by congenital or developmental malformations of BLOOD VESSELS, or reactive vascular proliferations, such as in bacillary angiomatosis. Angiomatosis is considered non-neoplastic.