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• Prenatal PBDE exposure can interfere with children's neurodevelopment. • PBDE blood concentrations in Canada are less than half those in the U.S.A. • We measured PBDEs in maternal blood and assessed IQ for 592 3-year-old children. • Prenatal PBDE exposure was related to lower IQ scores, but only among boys. • It suggests sex-specific neurotoxicity at the low PDBE levels in Canada. Prenatal exposure to polybrominated diphenyl ethers (PBDEs) has been associated with adverse neurodevelopmental outcomes in children, but evidence remains mixed regarding sex differences in this association. To examine the prospective association between prenatal PBDE exposure and cognitive ability in young children, as well as potential sex differences. The study was conducted in a multi-site Canadian pregnancy cohort recruited in 2008–11. PBDEs were measured in maternal plasma samples collected early in pregnancy. Cognitive ability was assessed using the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-III) in children at age 3 years (mean = 3.4). Multiple linear regression was used to analyze the association between maternal PBDE plasma concentrations (lipid-standardized and log10-transformed) and Verbal, Performance, and Full Scale IQ scores on the whole sample and stratified by sex, adjusting for confounders. The sample was composed of 592 children (291 boys and 301 girls). A tenfold increase in maternal blood PBDE concentration (sum of BDE-47, −99, −100, and −153) was associated with lower Full Scale scores in boys (−3.4 points; 95% CI: −7.0, 0.1), after adjusting for confounders. BDE-47 was the congener with the highest concentrations in maternal blood and a tenfold increase in exposure was associated with significantly lower Full Scale IQ scores in boys (−4.4 points; 95% CI: −7.9, −0.9), after adjusting for confounders. Verbal and Performance IQ scores were similarly associated with PBDE exposure. Maternal blood PBDE concentrations were not associated with IQ scores in girls. Prenatal exposure to background levels of PBDEs, especially BDE-47, was associated with lower IQ scores in boys, but not in girls. Our results support that exposure to PBDEs during early development may be sex-dependent and detrimental to a child's neurodevelopmental trajectory. [ABSTRACT FROM AUTHOR]