With the advent of IVF-ET, human oocytes and early embryos are now available for chromosomal analysis. In this study, 44 chromosomal preparations of human oocytes or early embryos from 36 women in an IVF program were analyzed. 13 oocytes with normal haploid chromosome complement were not fertilized. The cause of failure of fertilization was attributed to either male factors or to immaturity of oocytes. 4 oocytes not fertilized were found to be either hyperhaploid or hypohaploid. Among 26 fertilized but not cleavaged oocytes 8 were also abnormal, 7 were normal haploid but in different developmental stages with the fertilizing sperms, 11 showed polyspermia. One two-cell embryo showed normal diploid set. The high ratio of oocyte chromosomal abnormalities and abnormal fertilization may be explained by the use of drug hyperstimulation of follicular development. Similar abnormalities, although to a lesser extent, may occur in a natural reproductive process.