PP-29-101 Background/Aims: Dioxins (PCDDs, PCDFs, Dioxin-like PCBs) are endocrine disruptors, which when exposed to at low levels during pregnancy, are thought to have negative effects on fetal growth. Maternal dioxin concentrations vary greatly among individuals due to factors such as smoking, parity, and age. However, dioxin concentrations in maternal blood may also be affected by individual dioxin metabolizing ability. Dioxins are metabolized by the CYP1 enzyme family, which is produced by the binding of dioxins with the aromatic hydrocarbon receptor (AhR). Thus far, although 1 study has analyzed dioxin concentrations in relation to CYP1A1 genotype, no such studies have considered AhR. The aim of this study is to determine whether AhR polymorphisms affect dioxin concentrations in maternal blood. Methods: We examined 421 mothers who enrolled in our study between 2002 and 2005 in Sapporo, Japan. Relevant information was collected from a baseline questionnaire during pregnancy and from medical records at delivery. Dioxin concentrations in maternal blood were measured by high-resolution gas chromatography/high-resolution mass spectrometry. We used multiple linear regression analyses after adjustment for maternal age, height, pre-pregnancy weight, caffeine and alcohol intake during pregnancy, parity, smoking status during pregnancy, education level, annual household income, inshore and deep-sea fish intake during pregnancy, and blood sampling period. Results: When the difference in beta dioxin congener concentrations (log10 [pg/g lipid] scale) between the AhR (G>A, Arg554Lys) GA/AA referent genotype and GG genotype were measured, total non-ortho PCBs (beta = −0.044), and total mono-ortho PCBs (beta = −0.054) levels were significantly decreased. Specifically, 3,3′,4,4′-TeCB (#77), 3,3′,4,4′,5-PeCB (#126), 2′,3,4,4′,5-PeCB (#123), 2,3′,4,4′,5-PeCB (#118), 2,3,3′,4,4′-PeCB (#105), and 2,3′,4,4′,5,5′-HxCB (#167) levels were significantly decreased. Conclusion: This is the first report to analyze the relationship between xenobiotic metabolizing-enzymes and dioxins-congener concentrations in maternal blood. AhR polymorphisms may affect dioxin concentrations in maternal blood.