Nonradioactive in situ hybridization with an alpha-satellite DNA probe specific for chromosome 18 was performed on human interphase sperm nuclei to detect the frequency of sperm cells disomic for chromosome 18. A total of 16,127 sperm heads from eight healthy donors, aged 23-57 years, was investigated, and a minimum of 2000 sperm nuclei per proband was analyzed. The disomy rate ranged from 0.25% to 0.5%, with an average of 0.36%. This frequency does not differ significantly from that determined for other chromosomes.