Born March 20, 1912, in Baltimore, Klinefelter studied first at the University of Virginia, Charlottesville, and then attained his medical degree from Johns Hopkins School of Medicine.
Klinefelter worked at the Massachusetts General Hospital in Boston from 1941 to 1942; under the supervision of Fuller Albright he described a group of nine men with "gynecomastia, aspermatogenesis without aleydigism, and increased excretion of follicle-stimulating hormone", the first description of what would be called the Klinefelter syndrome.
[2] Initially he suspected this to be endocrine disorder and postulated the presence of a second testicular hormone, but in 1959, Patricia A. Jacobs and J.
Strong (Western General Hospital and University of Edinburgh) demonstrated that a male patient with the phenotype of Klinefelter syndrome had an additional X chromosome (47 XXY).
[4] Klinefelter served in the Armed Forces from 1943 to 1946 and then returned to Johns Hopkins where he remained during his professional life.