Abstract: To explore the efficiency of using aromatase inhibitors during luteal phase in in vitro fertilization IVF stimulated cycles for patients at high risk for ovarian hyperstimulation syndrome (OHSS). Methods: A total of 139 infertile women undergoing assisted reproductive technique with high risk for OHSS were enrolled in this clinical trial. In the treatment group 43 patients received five consecutive doses of aromatase inhibitors (letrozole) and support therapy combined with embryo cryopreservation. In the control group 96 patients received support therapy alone. All the patients were evaluated clinically, echographically, hematologically and tested for their steroid hormone. Results: There was significantly lower estrogen level in the treatment group 2, 5 and 8 days after oocyte retrieval compared with the control group (P<0.001), There was no significant difference in luteinizing hormone and progesterone levels 2, 5 and 8 days after oocyte retrieval in the treatment group and control group (P>0.05). There were 7 cases of severe OHSS in the treatment group and 18 cases of severe OHSS in the control group. The rate of severe OHSS was not significantly different in the treatment group and control group (P=0.12). No side effect was reported in either group. Conclusion: Treatment with letrzolein luteal phase decreases serum estrogen levels of patients after oocyte retrieval，but it couldn’t reduce the risk of severe OHSS.

Key words: SOvarian hyperstimulation syndrome, Aromatase inhibitors, Luteal phase, Fertilization in vitro