Objectives: The implications of the high Estradiol (E2) levels and the large number of oocytes in OHSS on the outcome of assisted reproduction is not clear in the world literature. Our objective is to study the oocyte maturity, fertilization rate (FR), implantation and pregnancy rates in severe OHSS.Design: Retrospective study with an age matched control group who were stimulated for in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) during the same period with the same stimulation protocol.Material and Methods: Group A = 42 patients who developed severe OHSS during ovulation induction for IVF or ICSI. They were diagnosed by the presence of ovarian enlargement more than 10 cm in diameter and massive ascites, diarrhea and abdominal pain. Group B = 42 patients, an age matched control group who received the same stimulation protocol and did not develop OHSS. Based on clinical, hormonal and ultrasonic examination group A was subdivided into group A1 = 31 patients with polycystic ovaries (PCO) and group A2 = 11 patients who were high responders with no PCO. On the same basis group B was similarly subdivided into group B1 = 5 patients and group B2 = 36 patients. Oocyte maturity was diagnosed by the extrusion of the first polar body in ICSI and by the morphological appearance of the cumulus-corona cell complex in IVF.Results: The mean E2 level, the mean number of oocytes per puncture, the percentage of mature oocytes, the FR were, 6700±2350 pg/ml, 25±8.5 62.7%, 41.5% in group A, and 2420±1540 pg/ml, 12.8±6.2, 76.4%, 59.4% in group B respectively. The difference was statistically significant in all items. The mean number of embryos per transfer, the implantation rate and the pregnancy rate were 3.9, 11%, 31% in group A and 3.7, 11.4%, 31% in group B. The difference was not significant between the 2 groups. In group A1, the mean number of oocytes, the percentage of mature oocytes and the fertilization rate were 27±8.1, 60% and 36% as compared to 21±6.6, 78 and 62% in group A2 respectively. The difference was statistically significant between all parameters.Conclusion: The lower oocyte maturity and quality resulted in a lower FR in the OHSS group. However, the final number of embryos was compensated by the larger number of oocytes. In group A, the quality of embryos that reached embryo transfer (ET) were not different from the control group as shown by the similar implantation and pregnancy rates in both groups. This also suggests that the extremely high levels of E2 in OHSS did not affect the endometrial receptivity. There was a highly significant difference in the percentage of PCO between groups A and B. This explains in part the reasons for the development of OHSS and its severity in group A, and the inferior quality of the oocytes in this group. Group A2 included high responders with no PCO. They produced significantly higher quality of oocytes and higher FR.