The objective of this study was to assess fertilization, implantation and pregnancy rates in infertile patients with severe teratozoospermia [P (poor prognosis) pattern sperm morphology assessed by strict criteria] treated by in-vitro fertilization (IVF) using a high insemination concentration (HIC), or by intracytoplasmic sperm injection (ICSI). This was a retrospective cohort study performed in an academic tertiary institution. The outcome of 115 consecutive ICSI cycles was compared to that of a similar number of cycles of IVF with HIC performed during a similar time frame and matched by woman's age and basal serum (cycle day 3) follicle stimulating hormone concentrations. The inclusion criteria were sperm morphology </=4% normal forms (P pattern) and >/=1x10(6) total motile spermatozoa per ejaculate. The diploid fertilization rate in the HIC-IVF group was 86% and in the ICSI group 68% (P < 0.05). Importantly, an equal number of embryos was transferred to both groups of patients. The morphological quality of the embryos (proportion of transfers having superior morphology embryo scores) was significantly better in the ICSI group than in the patients receiving HIC-IVF. Although there was a clear trend for better implantation and pregnancy rates in the ICSI group, these differences were not statistically significant. We conclude that, although HIC-IVF resulted in a higher fertilization rate than ICSI in patients with severe teratozoospermia, ICSI produced a significantly higher proportion of morphologically superior embryos with a tendency towards a higher implantation potential. Therefore, teratozoospermic patients having adequate numbers of motile spermatozoa should be offered ICSI as an alternative to modified (HIC) IVF treatment.