Embryos from a couple undergoing routine in-vitro fertilization for unexplained infertility had shown cleavage by day 1 in two consecutive cycles. The response of the woman to fertility drugs had been normal, and there were no known sperm abnormalities. In a subsequent cycle, accelerated cleavage occurred again and we used a modified method of spreading whole embryos and dual fluorescent in-situ hybridization (FISH) with directly labelled probes for chromosomes X and Y to determine if fertilization had occurred in these embryos. Nine oocytes were collected, five of which had cleaved when examined for pronuclei early on day 1 following late insemination. Pronuclei were observed in one of the remaining oocytes, but in this case, three were present. On day 2, all of the oocytes/embryos had cleaved and two were transferred to the patient. The remaining seven were spread for FISH analysis but nuclei were only obtained from five. In three, a Y signal was detected, indicating that fertilization had occurred. In all five embryos, a wide range of X chromosome signals were observed. These data suggest that the embryos had undergone abnormal fertilization and accelerated cleavage.