The results suggest that 47,XXY patients with an increased incidence of XY cells (average of 4.2 +/- 2.3) may have a higher probability of germ cells as we found sperm only in the ejaculate of Klinefelter syndrome patients with mosaic 46,XY cells (6.0 and 7.0%). On the other hand, 46,XY patients with mosaic sex chromosome aneuploidies detected by FISH analysis more often show symptoms of hypogonadism phenotypically resembling Klinefelter syndrome.