We evaluated the relationship between birth weight and mercury exposure levels in Seoul, Korea. We made a pregnant women s cohort, and followed the pregnancy outcome, between 2001-2005. Eighty-five pregnant women were recruited into this study after obtaining informed consent. Samples were collected at delivery from normal pregnant women who were living in the city of Seoul, Korea. Mercury concentrations in 85 sets of maternal and cord blood samples were measured using a gold-amalgam collection method. We used a multiple regression analysis to analysis the effect of mercury exposure on the birth weight. The mean levels of total mercury concentrations were 5.41(ppb) in maternal blood of pregnant women and 3.58(ppb) in umbilical cord blood. The mean concentration of umbilical cord blood mercury exposures was higher than the level recommended by WHO. There was a significant correlation between maternal and cord blood mercury concentrations. Mercury concentrations of umbilical cord blood was associated with birth weight. In addition, we found that mercury exposure may reduce the birth weight after adjusting for potential confounding factors. This study suggests that exposure to mercury concentration during pregnancy contributes to risk for birth weight. Therefore, prenatal and environmental education for various and possible sources of mercury exposure might be necessary for baby s good health. The finding of this study supports the construction of national policy for environmental health management.