Sperm samples from infertile men with oligozoospermia or teratozoospermia were studied by multicolour fluorescence in-situ hybridization (FISH) using DNA probes for chromosomes 13 and 21. A total of 90 809 sperm nuclei from nine infertile men and 182 799 sperm nuclei from 18 control donors were analysed. There was a highly significant increase in the frequency of spermatozoa disomic for chromosome 13 in infertile patients (0.28%) compared to control donors (0.13%) (two-tailed Z statistic P < 0.0001) and for chromosome 21 (0.48% in infertile men versus 0.37% in controls, P < 0.0001). Also there was a significantly increased frequency of diploid spermatozoa in infertile men (0.85%) compared to control donors (0.66%) (P < 0.0001). Our previous studies on these same infertile patients demonstrated increased frequencies of sperm disomy for chromosomes 1 and XY. This suggests that infertile men, who are prime candidates for intracytoplasmic sperm injection, may be at a very small increased risk of aneuploid offspring.