The effect of IMSI on avoiding fertilization with chromosomally abnormal sperm was examined in the present study. Normal shaped human sperm with large vacuoles were identified at 1,000× magnification on an inverted microscope with a 100×oil objective lens and were then cytogenetically analyzed after injection into mouse oocytes. In all semen samples from ten consenting patients receiving IVF treatment, more than 90% of the oval shaped sperm were determined to be abnormal, having vacuoles of various sizes on the head. Our cytogenetic analysis showed that the incidence of chromosome aberrations did not differ in sperm with large vacuoles from infertile patients and normal sperm from fertile donors. In addition, the risk of structural chromosome aberrations deriving from DNA lesions was very low in sperm with large vacuoles (9.1%) compared with elongated sperm (33.3%), the data for which was previously reported by us (2004). In conclusion, sperm DNA fragmentation dose not seem to explain the phenomenon reported by Berkovitz et al. (2006) in which injection of sperm with large vacuoles reduced pregnancy outcome. We also doubt whether IMSI is needed, since sperm with large vacuoles were easily identified at even 400 × magnification.