A total of 55 patients with various etiologies of infertility particitated in a study comparing two regimens of controlled ovarian hyperstimulation (COH) with GnRH agonists and gonadotropins. Nineteen patients were given an ultra-short stimulation protocol when the agonist was administered for 3 day from Day 2 of the cycle. The remaining 36 patients were given a long stimulation protocol when the agonist was administered from the mid-luteal phase of the cycle preceding the stimulation cycle. The mean number of gonadotropins used per patient was not different between two groups. No significant differences were found in the mean number of oocytes recovered, fertilization rate and embryo cleavage rate between two groups. Pregnancy and delivery rates were higher in ultra-short protocol than in long protocol, but these were not significant. These results suggest that an ultra-short protocol is as effective as a long protocol in in-vitro fertilization and embryo transfer.