Ovarian hyperstimulation dyndrome(OHSS) is one of the most serious complications of ovulation indution in infertile women particularly in poly cystic ovare (PCO).in recent years,in association with the increasing frequency of assisted reproductive technology(ART) usage and significant improvements in the treatment of infertility,this iatrogenic phenomenon is more prevalent.the aim of this clinical trial is prevention of severe forms of ovarian hyperstimulation dyndrome by reduction of the follicles.in this prospective study the patients with more than 5 years and different causes of infertility whowere referred for IVF to the Yazd infertility center from 1992 to 1995 were investigated.27 patients had the criteria of OHSS and agreed to continue their cycles,were selected.in this group,instead of cancellation of the cycle,about half of ovarian follicles of each patient were aspirated transvaginally by ultrasound guide and human chorionic Gonadotropin hormone(HCG) or Gonadotropin releasing hormone (GnRH) analogous was injected at the time of Luteinzing hormone (LH)surge.none of the 27 patients were found to have critical form of OHSS.clinical pregnancy were observed in 4 patients(14%).our preliminary results suggest that reduction of half of follicles may help to prevent the development of severe OHSS in high risk patients and prevention of the cancellation of the cycles.further research is needed to assess the potential of this novel approach.