BACKGROUND Abnormal semen parameters in chromosomally normal men are an indicator of an increased risk of sperm aneuploidy. Male carriers of chromosomal rearrangements may also display an increase in sperm aneuploidy for chromosomes not involved in the rearrangement, known as an interchromosomal effect (ICE), and this may be related to the impaired semen parameters of these men. METHODS Aneuploidy was examined in ejaculate sperm from 27 men: 8 carriers of chromosomal rearrangements with severe oligoasthenoteratozoospermia (OAT) or severe teratozoospermia; 10 chromosomally normal men with similarly abnormal semen parameters; and 9 proven fertile men with normal semen parameters. Fluorescence in situ hybridization was used to examine aneuploidy for chromosomes 13, 18, 21, X and Y. RESULTS We observed evidence of an ICE in three of the eight carriers of chromosomal rearrangements. However, men who were chromosomally normal but had severe OAT more frequently displayed increased disomy rates. Although autosomal disomy rates were only modestly increased in some of these men, increased XY disomy ranged from slight to extreme (up to a 100-fold increase). CONCLUSIONS Despite their similar semen parameters, infertile men with normal karyotypes displayed more frequent increases in sperm aneuploidy, particularly involving the sex chromosomes, than infertile men who were carriers of chromosomal rearrangements. The difference in the magnitude and type of sperm aneuploidy between the two infertile groups is likely related to the different causes of infertility.