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Introduction: A 33-year-old infertile patient presented with two early miscarriages in a spontaneous pregnancy and a pregnancy achieved by intrauterine insemination, respectively. She subsequently had nine failed IVF cycles. Despite the fact that >10 oocytes were retrieved in each cycle, all were characterized by abnormal fertilization demonstrated by the presence of only one pronucleus (IPN) and nonextraction of the secondnd polar body when examined 9-20 h following either standard insemination or intracytoplasmic sperm injection (ICSI). About 50% of these embryos further cleaved by day 3 post fertilization. While embryo transfer was performed in all cycles, no pregnancy was achieved. The aim was to analyse the genetic composition of oocytes and embryos in this patient in order to provide counselling regarding prognosis and treatment options. Materials/Methods: In her last cycle, ICSI was performed for all 10 oocytes. All first polar bodies (PB) were biopsied and analysed by fluorescence in-situ hybridization (FISH) probes for chromosomes 13, 18, 21, X and Y. In addition, all cleaving embryos were subjected to blastomere biopsy, followed by FISH analysis, as above. Since none of the embryos were found to be euploid, they were further subjected to single cell PCR analysis using a panel of highly polymorphic markers from different chromosomes (6, 7, 17, 19, X and Y). Results: FISH analysis of the first PB demonstrated normal segregation of chromosomes in meiosis 1. FISH analysis of single blastomeres from five cleaving embryos demonstrated three diploid and two mosaic triploid embryos. However, haplotype analysis demonstrated that all embryos including the diploid ones, had two sets of maternal alleles. Conclusion: We conclude that the diploid embryos represent parthenogenetic activation of a diploid oocyte. Furthermore, since ICSI was performed, the triploid embryos must have resulted from fertilization of a diploid oocyte by a single spermatozoon. Thus the abnormality detected in this case results from failure to extrude the second PB. Although abnormality of the spermatozoa has not been ruled out, this most likely represents failure in the oocyte activation machinery leading to incompetence of the oocyte to complete meiosis II as well as to decondense the sperm nucleus into the male pronucleus. [ABSTRACT FROM AUTHOR]