['To evaluate the prognostic significance of low serum E2 concentrations in controlled ovarian hyperstimulation (COH) cycles for IVF.', 'Retrospective study.', 'Assisted conception unit of a university hospital.', 'One thousand four hundred and forty patients undergoing COH for IVF.', 'COH, serum E2 measurement, ultrasonographic scanning of ovarian follicles, oocyte retrieval, and ET.', 'Cancellation and pregnancy rates.', 'Patients were classified into four groups according to serum E2 levels on the sixth day of COH: group A (E2 level < 50 pg/mL [114 cycles]), group B (E2 level 51-100 pg/mL [189 cycles]), group C (E2 level 101-200 pg/mL [320 cycles]), and group D (E2 level >200 pg/mL [817 cycles]). Group A experienced the highest cancellation rates (65.1%) and lowest pregnancy rates (7.8%) despite requiring significantly more hMG ampules (47.8+/-1.7). The cancellation rate was higher (75.1%) and no pregnancy occurred in a subset of group A in whom COH was initiated with > or =3 ampules (225 IU) of gonadotropins.'] Are low serum estradiol concentrations after five days of controlled ovarian hyperstimulation for in vitro fertilization associated with poor outcome?