A chromosomal complement of 227 human oocytes was studied to provide information on the frequency and type of chromosomal abnormalities in oocytes failing in vitro fertilization.Normal haploid chromosome complement was found in 54.6%; chromosomal abnormalities consisting of diploid sets were identified in 16.7% and aneuploidy was observed in 26%. Premature condensation of sperm chromosomes of the GI-phase was observed in 22.9% oocytes. Male infertility was correlated with an increase in the rate of aneuploidy when compared with tubal infertility. The rate of chromosome abnormalities for the oocytes recovered from women who had no fertilized oocytes was significantly higher compared to those with at least one oocyte fertilized.A high frequency of chromosome abnormalities in unfertilized oocytes suggests that natural selection against chromosome abnormalities may occur even prior to fertilization.