Abnormalities of fertilization resulting in suppression of second polar body formation, activation without sperm fusion and multiple sperm fusion occur frequently following assisted (intracytoplasmic sperm injection; ICSI) and IVF in the human. Their genetic status was investigated by multiprobe fluorescence in-situ hybridization (FISH) or polymerase chain reaction (PCR) on sibling blastomeres. IVF embryos with single pronuclei at the zygote stage were usually diploid and fertilized. Embryos derived from IVF with single pronuclei can be safely replaced back to the patient. These embryos develop following formation of a single or aggregated pronucleus, a process found to occur in sea urchins. Single pronucleate ICSI zygotes are usually activated but not fertilized. The parental status of individual pronuclei was investigated in dispermic embryos. It was found that the distal pronucleus was usually male in origin and that the sex ratio was restored in enucleated zygotes, however, dispermic embryos become mosaic. Genetic heterogeneity was not restored in enucleated dispermic embryos; none of them became truly diploid. Mosaicism, on the other hand, was not common among digynic ICSI embryos and any such mosaicism originated at the third cleavage division, a pattern which is similar in mosaic monospermic embryos. Most of the digynic embryos were triploid, indicating that the first division was bipolar in origin. Most digynic embryos from which a female pronucleus was removed became diploid and their genetic condition was considered normal. From this work it is concluded that the sperm centriole is active in the human oocyte, rendering most monospermic embryos, including those that are digynic, non-mosaic. Removal of a single male pronucleus will not revert dispermic embryos to a normal status because of the activity of extra sperm centrioles. Transfer of enucleated dispermic embryos or their use as models for embryonic development should be reconsidered.