The method of simultaneous three-chromosome fluorescence in situ hybridization (FISH) was developed using repetitive DNA sequence probes for chromosomes 8, X and Y and applied to semen of 14 men from two healthy groups who differed in their average ages (46.8 +/- 3.1 years, n = 4 v. 28.9 +/- 5.0 years, n = 10). The frequencies of disomic sperm determined by FISH compared well with frequencies obtained using the hamster-egg technique for human-sperm cytogenetics and with the frequencies of disomic and diploid sperm reported in previous FISH studies in this laboratory. The two groups of men did not differ in their baseline frequencies of sperm disomic for chromosome 8 (approximately 6.5 per 10(4) sperm), sperm with XY8 aneuploidy (approximately 9.5 per 10(4) sperm), or sperm with autodiploidy XX88 or YY88 (approximately 2 per 10(4) sperm). However, the older group had statistically higher frequencies of sperm carrying sex chromosomal disomy than the younger group (5.1 v. 2.2 per 10(4) sperm for XX8; 5.9 v. 2.0 per 10(4) sperm for YY8; P < 0.005). A recent report from this laboratory of sex-chromosomal aneuploidy in sperm of aged mice provides inter-species corroborating evidence for this preliminary finding of a paternal age effect on sperm aneuploidy in human males.