OBJECTIVE How much variation in oocyte yield after controlled ovarian stimulation (COS) can be accounted for by known patient and treatment characteristics? CONCLUSIONS There is substantial variation in the COS responses of similar women and in repeated COS episodes undertaken by the same woman, which cannot be accounted for at present. BACKGROUND The goal of individualized COS is to safely collect enough oocytes to maximize the chance of success in an ART cycle. Personalization of treatment rests on the ability to reduce variation in response through modifiable factors. Multilevel modelling of a routine ART database covering the period 1 October 2008-8 August 2012 was employed to estimate the amount of variation in COS response and the extent to which this could be explained by immutable patient characteristics and by manipulable treatment variables. A total of 1851 treatment cycles undertaken by 1430 patients were included. The study was not subject to attrition, as cancelled cycles were included in the analysis. METHODS Women aged 21-43 years undergoing ovarian stimulation for IVF (possibly with ICSI) using their own eggs at a tertiary care centre. RESULTS Substantial unexplained variation in COS response (oocyte yield): was observed (3.4-fold (95% CI: 3.12 to 3.61)). Only a relatively small amount of this variation (around 19%) can be explained by modifiable factors. A significant, previously undescribed predictor of response was the practitioner performing oocyte retrieval, with 1.5-fold variation between surgeons with the highest and lowest yields. CONCLUSIONS Although a large number of covariables were adjusted for in the analysis, including those that were used for dosing and determination of the stimulation regimen, this study is subject to confounding due to unmeasured variables and measurement error. CONCLUSIONS The present study suggests that there are limits to the extent that COS response can be predicted on the basis of known factors, or controlled by manipulation of treatment factors. Moreover, modifiable variation in response appears to be partially attributable to differences between surgeons performing oocyte retrieval. Consequently, consistent prevention of ineffective or unsafe responses to COS is not likely to be possible at present. Our results highlight the importance of blinding surgeons in RCTs. The data also suggest that there is likely to be limited scope for personalized treatment unless additional predictors of ovarian response can be identified. BACKGROUND J.W. is funded by a Doctoral Research Fellowship from the National Institute for Health Research (DRF-2014-07-050) supervised by S.A.R. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health. J.W. is a statistical editor of the Cochrane Gynaecology and Fertility Group. S.A.R. is a statistical editor for Human Reproduction. J.W. also declares that publishing peer-reviewed articles benefits his career. A.L.M. has received consultation fees from MSD, Merck Serono, Ferring, TEVA, Roche, Beckman Coulter.