Mercury (Hg) is toxic to both the reproductive and nervous systems. In addition, glutathione S-transferases (GSTs), which conjugate glutathione to a variety of electrophilic compounds, are involved in the detoxification of Hg.In this study we examined the association between prenatal exposure to Hg and birth weight as well as the influence of GST polymorphisms.The total Hg concentration in maternal and cord blood was measured from 417 Korean women and newborns in the Mothers and Children's Environmental Health study from 2006 to 2008. Information on birth weight was collected from the patients' medical records. The genotyping of glutathione S-transferase M1 (GSTM1) and glutathione S-transferase T1 (GSTT1) polymorphisms was carried out using polymerase chain reaction. Regression analysis was performed to determine the association between the blood Hg concentration and birth weight in mothers with GSTM1 and GSTT1 polymorphisms.The geometric mean levels of Hg in the maternal blood during late pregnancy and in cord blood were 3.30 microg/L and 5.53 microg/L, respectively. For mothers with the GSTT1 null genotype, elevated Hg levels in maternal blood during late pregnancy were associated with an increased risk of lower birth weight. For mothers with both GSTM1 and GSTT1 null genotype, both maternal and cord blood Hg levels were associated with lower birth weight.This study suggests that the interactions of Hg with GSTM1 and GSTT1 polymorphisms play a role in reducing birth weight.