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Consistent reports are not available on the effects of dioxin-like polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDD)/ polychlorinated dibenzofurans (PCDF) (dioxin-like compounds [DLCs]) on child neurodevelopment. Further, the effect of background-level exposure to individual DLC isomers is not known.We carried out the Sapporo cohort study to evaluate the effect of prenatal exposure to each DLC isomer on child neurodevelopment at 6 and 18 months of age, and assessed sex-specific differences in these effects.The levels of all and each individual DLC isomers were estimated in maternal peripheral blood. Neurodevelopment was evaluated using the Bayley Scales of Infant Development-2nd Edition for 6-month-old infants (n = 190) and 18-month-old children (n = 121).In male children, levels of 10 DLC isomers were significantly negatively associated with the Psychomotor Developmental Index (PDI) at 6 months of age after adjustment for potential confounding variables. However, at 18 months of age, these associations were absent. In female children, the level of only one DLC isomer was significantly negatively associated with PDI at 6 months of age. However, in contrast to the male children, the levels of six DLC isomers in 18-month-old female children were significantly positively associated with the Mental Developmental Index.These findings indicate that adverse neurodevelopmental effects of prenatal background-level exposure to DLCs may be stronger in male children.Adverse developmental effects of prenatal exposure to DLCs may be stronger in male.A significant negative association to development of DLCs was absent at 18 m of age.Some of DLCs were positively associated with MDI at 18 m of age in female.