Objective: To study whether reported, but inconsistent, associations between the FMR1 CGG repeat lengths in the intermediate, high normal, or low normal range differentiate women diagnosed with diminished ovarian reserve (DOR) from population controls and whether associations vary by race/ethnic group. Design: Case-control study. Setting: Academic and private fertility clinics. Patient(s): DOR cases (n = 129; 95 Whites, 22 Asian, 12 other) from five U. S. fertility clinics were clinically diagnosed, with regular menses and no fragile X syndrome family history. Normal fertility controls (n = 803; 386 Whites, 219 African-Americans, 102 Japanese, 96 Chinese) from the United States-based SWAN Study had one or more menstrual period in the 3 months preenrollment, one or more pregnancy, no history of infertility or hormone therapy, and menopause R46 years. Previously, the SWAN Chinese and Japanese groups had similar FMR1 CGG repeat lengths, thus they were combined. Intervention(s): None. Main Outcome Measure(s): FMR1 CGG repeat lengths. Result(s): Median CGG repeats were nearly identical by case/control group. DOR cases had fewer CGG repeats in the shorter FMR1 allele than controls among Whites, but this was not significant among Asians. White cases had fewer CGG repeats in the shorter allele than Asian cases. No significant differences were found in the high normal/intermediate range between cases and controls or by race/ethnic group within cases in the longer allele. Conclusion(s): This study refutes prior reports of an association between DOR and high normal/intermediate repeats and confirms an association between DOR and low normal repeats in Whites. (C) 2016 by American Society for Reproductive Medicine.