Abstract BACKGROUND Cognitive status in females with mutations in the FMR1 (fragile X mental retardation 1) gene is highly variable. A biomarker would be of value for predicting which individuals were liable to develop cognitive impairment and could benefit from early intervention. A detailed analysis of CpG sites bridging exon 1 and intron 1 of FMR1, known as fragile X–related epigenetic element 2 (FREE2), suggests that a simple blood test could identify these individuals. METHODS Study participants included 74 control females (<40 CGG repeats), 62 premutation (PM) females (55–200 CGG repeats), and 18 full-mutation (FM) females assessed with Wechsler intelligence quotient (IQ) tests. We used MALDI-TOF mass spectrometry to determine the methylation status of FREE2 CpG sites that best identified low-functioning (IQ <70) FM females (>200 CGG repeats), compared the results with those for Southern blot FMR1 activation ratios, and related these assessments to the level of production of the FMR1 protein product in blood. RESULTS A methylation analysis of intron 1 CpG sites 10–12 showed the highest diagnostic sensitivity (100%) and specificity (98%) of all the molecular measures tested for detecting females with a standardized verbal IQ of <70 among the study participants. In the group consisting of only FM females, methylation of these sites was significantly correlated with full-scale IQ, verbal IQ, and performance IQ. Several verbal subtest scores showed strong correlation with the methylation of these sites (P = 1.2 × 10−5) after adjustment for multiple measures. CONCLUSIONS The data suggest that hypermethylation of the FMR1 intron 1 sites in blood is predictive of cognitive impairment in FM females, with implications for improved fragile X syndrome diagnostics in young children and screening of the newborn population.