This study was undertaken to establish baseline data on the chromosomal status of 'failed-fertilized' oocytes derived from in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) procedures. A cytogenetic analysis was undertaken on 162 IVF and 51 ICSI oocytes. In all, 82.1% (133/162) of the IVF and 78.4% (40/51) of the ICSI oocytes had metaphase II (MII) plates, of which 50.4% of the IVF and 47.5% of the ICSI oocytes were analysed further. Chromosomes of the G-group (21-22) were identified with the majority of the anomalies. No overall significant difference in the aneuploidy rate was found for the IVF (37.3%) of ICSI (31.6%) oocytes, or with maternal age. However, chromosome anomalies, e.g. diploidy, fragmented and broken chromatids, single sperm and oocyte chromatids, were found in oocytes from IVF patients aged > 36 years and in the ICSI oocytes throughout the maternal age range (31-38 years). The status of the polar body chromatin indicated that there was no overall significant difference in the maturation of the IVF and ICSI oocytes. Evidence of successful sperm delivery was found in 72.5% (37/51) of the ICSI failed-fertilized oocytes. In this group there was a significant increase in the incidence of premature chromosome condensation: 19.6% (10/51) contained sperm chromosomes, 7.8% (4/51) had swollen sperm heads, and the remaining 45.0% had condensed sperm heads. The presence of both sperm and MII oocyte chromosomes was found in 19.6% (10/51) of the ICSI and 8.6% (14/162) of the IVF failed-fertilized oocytes. Specific fluorescent in-situ hybridization DNA probes were used to re-analyse the chromosomes of karyotyped 'failed-fertilized' IVF oocytes and, for the first time, applied to the karyotyped chromosomes of failed-fertilized ICSI oocytes. The hybridization efficiency was 86-95% for the centromere probe and 100% for probes 21 and 18.