No / BACKGROUND: The possibility that oligozoospermic men may have elevated levels of genetic damage in their sperm is of particular concern as they could transmit defects to their offspring. METHODS: Sperm samples were obtained from 12 infertile, oligozoospermic patients and 12 healthy normozoospermic volunteers. Fluorescence in-situ hybridization (FISH) was used to determine aneuploidy rates in sperm and inverse restriction site mutation (iRSM) assay to determine gene mutations; defective chromatin packaging was quantified by sperm chromatin structure assay (SCSA) and DNA strand breaks by the Comet assay. RESULTS: FISH analysis showed a significant increase in gonosomal X,Y,18 (P < 0.01) disomy and diploid sperm with X,Y,18,18 (P < 0.05) in the infertility patients compared with the controls. A significant increase (P < 0.01) in disturbed sperm chromatin was found in the infertility patients compared with the control group using the SCSA assay. In the Comet assay, a significant increase (P < 0.01) in the tail moment was found in the infertility patients compared with the control group, indicating significantly high levels of DNA strand breaks. There was no significant increase in point mutations detected by iRSM assay. CONCLUSIONS: The data indicate that infertile oligozoospermic men have an elevated level of XY aneuploidy and XY diploidy in the germ-line, as well as elevated levels of sperm chromatin disturbances and sperm DNA strand breaks. These data demonstrate that oligozoospermic infertility patients show several different types of genetic damage in their sperm. Thus, such men appear to have defects at a variety of levels of spermatogenesis and their infertility may not just be a result of the oligozoospermia.