In vitro fertilization and embryo transfer is now an established treatment for certain forms of human infertility. High success rates have been reported following oocyte recovery and fertilization in vitro, the only major remaining difficulty being the high failure rate (80%) of implantation after replacement of the embryo into the mother. One possible reason for this failure of implantation is that some embryos may carry a lethal chromosome abnormality which does not allow development beyond the preimplantation stage. The need for information on the chromosomal status of early embryos fertilized and developed in vitro is recognized but two reported attempts to examine them have met with technical problems. Here we describe a method for examining chromosomes in 8-cell human embryos. Although we have made complete chromosome analyses for only three oocytes, our findings are surprising in that two of the three embryos were chromosomally abnormal. Data on the DNA content of the nuclei in a further eight cases showed that approximately 20% overall may be haploid.