To research the origin of nondisjunction, 200 human oocytes that had failed to be fertilized during in vitro fertilization procedures were examined. These oocytes had completed meiosis I and were arrested in metaphase II (MII). The majority had a normal MII metaphase complement, showing 23 chromosomes, each consisting of two sister chromatids joined at a common centromere. The remaining oocytes all had abnormal chromosome compositions. Surprisingly, when trisomy was considered, none of the abnormal oocytes had 24 chromosomes.