The majority of early spontaneous abortions carry a lethal chromosomal anomaly. While it is recognized that several factors would be responsible for some IVF failures, it is important to determine the contribution of chromosomal aberrations to the preimplantation loss of embryos produced in vitro. Chromosome analysis of embryos not destined for replacement in the uterus could help to elucidate this phenomenon of early embryonic loss. Fifty-five out of 239 embryos fertilized in vitro were successfully karyotyped and amongst these the overall rate of diploidy was 25.5% in this study, which mainly comprised rejected embryos. Embryos without cleavage had mostly a chromosomal defect (20/38) and only a minority (9/38) were unfertilized. Numerical abnormalities were found in a total of 33/46 (71.7%) morphologically normal embryos. In contrast a diploid chromosomal complement was found in only 11.1% (1/9) of morphologically abnormal embryos.