ntroduction: The effect of Body Mass Index (BMI), which reflects the woman’s obesity, in IVF treatment cycle, remains unclear. In 1997, the World Health Organization (WHO) provided authoritative refinements to the overweight terminology and BMI cutoffs [1]. Objective: To verify the relationship between BMI and ovarian response in IVF treatment cycle. Design: Retrospective study. Materials & Methods: The study includes 2625 IVF treatment cycles performed in our IVF center in the period of 4 years. Patients were divided into five groups using the WHO criteria according to their BMI [2]. Cancellation rate, mean last E2 before hCG administration, mean endometrial thickness, mean duration of stimulation, number of eggs retrieved, fertilization rate, pregnancy and abortion rates were analyzed. The unpaired t-test was used in statistical analysis. Results: There was statistically significant less mean oestradiol level prior to hCG, less endometrial thickness and less number of simulation days as BMI gets higher. In contrast, there was a positive relationship between cancellation rate and higher BMI except with BMI >39 which was not, possibly due to lower number of patients available. But if we look at the cause of cancellation it was 100% due to insufficient number of follicles obtained for this group (BMI >39). Also, days of stimulation are significantly lower for the same group of patients in comparison with the other groups. Retrieval, fertilization and pregnancy rates were not significant between all groups. Abortion rate gets significantly higher as BMI increased. Conclusion: