We report on a patient with Klinefelter syndrome (KS) and the homogeneous aneuploidy 47,Xi(Xq)Y, or male trisomy Xq. He had many characteristics of classical KS: small testes, azoospermia, elevated FSH and LH, average intelligence, and normal androgenization, but his stature was not increased, compared with his father's and brothers'. The i(Xq), found in all cells analyzed, was late-replicating, monocentric, and also asymmetric for the RBG-banding of the two arms, indicating a different chronology of DNA synthesis in each arm. When indicated, in the seven previously reported cases, the level of plasma testosterone was always subnormal; it was normal (650 ng/100 ml) in our patient, who had normal masculinization. Thus the level of testosterone among patients with KS is not necessarily lower with an extra Xq. Furthermore, the sharp contrast in the height of KS patients with or without an i(Xq) is striking. It appears definitely possible to associate the isochromosome Xq Klinefelter syndrome with a lack of height increase.