In this study we have investigated the arrangement of sex chromosomes in sperm from two severe oligozoospermic patients, apparently affected by the classic form of Klinefelter's syndrome (KS). Multicolor fluorescence in situ hybridization has been used to recognize chromosomes X, Y, and 8 in sperm from patients and 10 fertile men with normal 46,XY karyotype. In patients affected by KS, we detected important numerical sex chromosome abnormalities (approximately 20%). In all normal fertile men, X- and Y-bearing spermatozoa were present in a 1:1 ratio. On the contrary, in our patients the frequency of 23,Y-bearing sperm was strongly reduced compared with that of both 23,Y sperm in the controls and 23,X sperm in the same subject affected by KS, resulting in a 23,X-/23,Y-bearing sperm ratio of 2:1. Moreover, the frequency of 24,XY disomic sperm was significantly higher in the absence of the 22,0 hypoaploidy expected from a common origin from a nondysjunction during the first meiosis in a normal 46,XY cell. In conclusion, the results of the present study demonstrate a peculiar distribution of sex chromosomes in sperm from two patients with KS, in agreement with the hypothesis that 47,XXY germ cells are able to complete the meiotic process by producing mature spermatozoa.