OBJECTIVES: The purpose of this study was to monitor blood lead in a northern Swedish cohort of mothers and children during pregnancy and at birth. METHODS: Blood lead was analyzed during pregnancy and in the umbilical cords of 290 women living near a smelter and in 194 control subjects. RESULTS: During pregnancy, there were statistically significant overall increases in blood lead concentrations by 20% and 15% in the smelter and reference areas, respectively. Mean maternal blood lead concentrations at delivery were 0.15 mumol/L (3.11 micrograms/dL) in the smelter area and 0.13 mumol/L (2.69 micrograms/dL) in the control area. Umbilical cord blood lead levels were 80% to 87% of the maternal levels. Blood lead levels were influenced by place of residence, employment at the smelter, smoking, and wine consumption. Maternal serum calcium levels decreased during pregnancy and were significantly lower than those of the newborns. CONCLUSIONS: An increase in blood lead concentrations was found during pregnancy, despite increased blood volume and unchanged or decreasing environmental lead levels. The mobilization of lead from bone during pregnancy may explain the increase.