Ovarian stimulation in IVF allows selection of embryos for transfer, but may have detrimental effects on oocyte and embryo quality and endometrial receptivity. This study investigated the optimal response to ovarian stimulation in terms of number of oocytes for achieving pregnancy in a first IVF cycle. Data from 7422 women who underwent their first IVF cycle for standard indications were analysed. All had been treated with exogenous gonadotrophins and gonadotrophin releasing hormone (GnRH) agonist co-treatment in a long down-regulation protocol between 1990 and 1995. Pregnancy rates in relation to the number of obtained oocytes were adjusted for age, fecundity, subfertility cause, gonadotrophin dosage, type of luteal support, and number of transferred embryos by multivariate analysis. Of the 7422 women who underwent oocyte retrieval, overall 85% had an embryo transfer and 24% conceived. The highest pregnancy rates per embryo transfer and per started cycle were observed when 13 oocytes were obtained (31 and 28%, respectively). This study supports the concept of an optimal range of oocytes obtained in response to ovarian stimulation for IVF, below and above which outcomes are compromised.