The aim of this study was to determine the incidence of sex chromosome aneuploidy in spermatozoa of two males with a 47,XYY karyotype and one male with a 46,XY/47,XYY constitution. Spermatozoa obtained from two oligospermic patients and one volunteer semen donor were studied by multicolour fluorescence in situ hybridization. In the XY/XYY male, the frequencies of X-bearing to Y-bearing sperm were significantly different from the 1 : 1 expected ratio. Significantly increased frequencies were found in the mosaic and non-mosaic males for 24,XX and 24,YY sperm when compared with control donors. The number of 24,XY sperm was significantly different from the controls in the XYY males, but not in the mosaic male. The incidence of disomy 18 and the rate of diploidy also increased in the three patients. However, the mosaic male had the lowest cumulative rate of disomic and diploid spermatozoa when compared with the two XYY patients. Our data suggest that: (i) chromosome abnormalities observed in spermatozoa of the two XYY oligoasthenoteratospermic (OAT) males arise through segregation errors in XY germ cells rather than normal meiosis of XYY germ cells, (ii) mosaic XYY males with normal semen parameters have a lower risk of producing offspring with a sex chromosomal abnormality than OAT males with XYY karyotype.