Since rare cases of sex chromosome anomalies such as XXX and XXY were observed in the offspring of our XO breeder mice, we performed a cytogenetic analysis of metaphase II oocytes of XO mice to determine whether any changes in chromosomal configurations occur. We found a significantly increased incidence of unpartnered single chromatids in metaphase II oocytes of XO mice. Such single chromatids may contribute to embryonic aneuploidy. In addition, the tendency of the X-chromosome to segregate non-randomly to the oocyte rather than to the polar body was confirmed.