['To determine whether the high lipid content of human follicular fluid influences oocyte maturation.', 'Mouse oocytes as substitutes for human oocytes were exposed to follicular fluids of differing lipid content with outcome monitoring.', 'Private infertility clinic and university laboratory.', 'Seventy-four women seeking assisted reproduction, and gonadotropin-stimulated mice.', 'Assay of follicular fluids for triglyceride and free fatty acids, and stimulation of mouse cumulus-oocyte complexes (COCs) to maturity in vitro in the presence of lipid-rich or lipid-poor follicular fluid.', 'Oocyte lipid content, expression of endoplasmic reticulum stress marker genes, and oocyte maturation assessed in mouse COCs exposed to lipid-rich follicular fluid were compared with complexes exposed to lipid-poor follicular fluid and complexes matured in vivo.', 'Follicular fluids were obtained from women of known body mass index undergoing oocyte aspiration at a private infertility clinic, and the follicular fluids were assayed for triglyceride and free fatty acids; those with the highest and lowest levels of these lipids were selected. The mouse COCs exposed to lipid-rich follicular fluid during their maturation had increased oocyte lipid content, induction of endoplasmic reticulum stress markers, and impaired oocyte nuclear maturation.'] Is exposure to lipid-rich follicular fluid associated with endoplasmic reticulum stress and impaired oocyte maturation in cumulus-oocyte complexes?