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Exposure to perfluoroalkyl substances (PFAS) has been shown to be neurotoxic in experimental studies, but epidemiological evidence linking prenatal PFAS exposure to child neurodevelopment is equivocal and scarce. To quantify associations between prenatal exposure to legacy PFAS and children's intelligence (IQ) and executive functioning (EF) in a Canadian pregnancy and birth cohort and to determine if these associations differ by child sex. We measured first-trimester plasma concentrations of perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and perfluorohexanesulfonic acid (PFHxS) in the Maternal-Infant Research on Environmental Chemicals (MIREC) study and assessed children's full-scale (n = 522), performance (n = 517), and verbal (n = 519) IQ using the Wechsler Preschool and Primary Scale of Intelligence (WPPSI-III). Children's working memory (n = 513) and ability to plan and organize (n = 514) were assessed using a parent-reported questionnaire, the Behavior Rating Inventory of Executive Function - Preschool Version (BRIEF-P). We quantified associations between individual log2-transformed PFAS exposure and children's IQ and EF using multiple linear regression analyses and evaluated effect modification by child sex. We also used Repeated Holdout Weighted Quantile Sum (WQS) regression models with effect modification by child sex to quantify the effect of combined exposure to all three PFAS chemicals on IQ and EF. All models were adjusted for key sociodemographic characteristics. Geometric mean plasma concentrations (IQR) for PFOA, PFOS and PFHxS were 1.68 (1.10–2.50), 4.97 (3.20–6.20) and 1.09 (0.67–1.60) μg/L respectively. We found evidence of effect modification by child sex in all models examining performance IQ (p <.01). Specifically, every doubling of PFOA, PFOS, and or PFHxS was inversely associated with performance IQ, but only in males (PFOA: B = −2.80, 95% CI: −4.92, −0.68; PFOS: B = −2.64, 95% CI: −4.77, −0.52; PFHxS: B = −2.92, 95% CI: −4.72, −1.12). Similarly, every quartile increase in the WQS index was associated with poorer performance IQ in males (B = −3.16, 95% CI: −4.90, −1.43), with PFHxS contributing the largest weight to the index. In contrast, no significant association was found for females (B = 0.63, 95% CI: −0.99, 2.26). No significant associations were found for EF in either males or females. Higher prenatal PFAS exposure was associated with lower performance IQ in males, suggesting that this association may be sex- and domain-specific. • We assessed associations between PFAS and child neurodevelopment. • PFOS, PFOS and PFHxS were inversely associated with nonverbal IQ in males. • In mixture models, PFHxS was identified as a chemical of concern. • We observed no associations between PFAS and executive functioning. • The association between PFAS and IQ may be sex and domain-specific. [ABSTRACT FROM AUTHOR]