A major side-effect of controlled ovarian stimulation (COS) in patients with polycystic ovarian syndrome (PCOS) is the risk of ovarian hyperstimulation syndrome (OHSS). In-vitro maturation (IVM) of immature oocytes represents a potential alternative for the fertility treatment of these patients. Two patients at high risk of OHSS were primed with 10,000 IU HCG 36 h before oocyte retrieval. After retrieval, oocyte maturity was evaluated. Oocytes considered to be mature at the time of collection were inseminated by IVF or ICSI, and the resulting embryos were cultured to blastocysts. Transfer of these blastocysts resulted in pregnancy in both patients. Immature oocytes were cultured in YS medium supplemented with 30% human follicular fluid, 1 IU/ml rFSH, 10 IU/ml HCG and 10 ng/ml epidermal growth factor (rhEGF). After in-vitro maturation of the oocytes, ICSI was performed. Two and five expanded blastocysts were obtained after 5 day culture and were cryopreserved. This report indicates that mature oocytes can be collected at the time of retrieval using only in-vivo HCG priming in women with PCOS, and clinical pregnancy can be established by transfer of blastocysts derived from the mature oocytes. This approach opens a potential for a new dimension in the management of patients with PCOS.