The present study examined whether the rates of oocyte maturation, fertilization and development, as well as pregnancy rate could be improved by human chorionic gonadotrophin (HCG) priming 36 h before immature oocyte retrieval in patients with polycystic ovarian syndrome (PCOS). Immature oocyte retrieval was performed on day 10-14 of the cycles and patients were randomly allocated either to be primed with 10 000 IU of HCG before the retrieval, or not primed. Immature oocytes were cultured for 24-48 h in TC-199 medium with 20% (v/v) inactivated fetal bovine serum (FBS) supplemented with 75 mIU/ml follicle stimulating hormone (FSH) and luteinizing hormone (LH). Intracytoplasmic sperm injection (ICSI) was performed in all mature oocytes and the resulting embryos were transferred on day 2 or 3 after ICSI. A total of 17 patients underwent 24 completed treatment cycles. Thirteen cycles were primed with HCG and 11 other cycles were not primed. The mean number of oocytes retrieved was comparable in the two groups (7.8 +/- 3.9 versus 7.4 +/- 5.2). The percentage of oocytes achieving maturation at 48 h was significantly higher (P < 0.05) in the HCG-primed group (84.3%, 86/102) than in the non-HCG-primed group (69.1%, 56/81). Oocyte maturation was hastened in the HCG-primed group. Following 24 h of culture, 78.2 +/- 7.1% of oocytes were matured in the HCG-primed group compared with 4.9 +/- 2.5% of oocytes in the non-HCG-primed group (P < 0.001). There were no significant differences in the rates of oocyte fertilization and cleavage in these two groups. There were five clinical pregnancies (38.5%) in the HCG-primed group, and three pregnancies (27.3%) in the non-HCG-primed group.