To determine the degree of chromosomal aberrations in the sperm of men with hepatitis C. 36 subjects (20 in the healthy control group and 16 in the HCV infection group [genotype 1b]) were recruited. The cause of viral transmission was unknown in all patients. Sperm samples from the subjects were used for interspecies in vitro fertilization of zona-free golden hamster ova. The frequencies of spermatozoan aberrations were compared between the healthy control group and the HCV infection group. A total of 280 sperm chromosome complements were studied, including 129 complements from the 16 donors in the HCV infection group and 151 from the healthy control group. Of the 129 analyzable sperm metaphase spreads in the HCV infection group, 14 (10.85%) complements contained chromosomal aberrations, which was significantly higher than the number (9/151, 5.96%) in the healthy control group (p < 0.01). Moreover, in the HCV infection group, chromosomes frequently showed anomalies such as stickiness, clumping, and failure to stain, which prevented their analysis. HCV infection has mutagenic effects on the chromosomes in sperm and may lead to extensive heredi-tary effects owing to genetic alterations and/or chromosomal aberrations. In addition, there is the possibility of vertical transmission of HCV via the germ line.