Recent longitudinal studies with human subjects and nonhuman primates using high-precision stable lead isotopes show that lead is mobilized from the maternal skeleton during pregnancy and the postpartum period. We have now calculated the cumulative lead release (lead flux in micrograms) mobilized from the skeleton during these periods by means of analysis of monthly PbB samples from recent immigrants to Australia. Results included a statistically significant inverse relationship (P = .006) between the lead flux and the time of conception after the arrival of the subjects in Australia. By using an area-under-the-curve approach to determine the added lead inputs to blood during pregnancy and nursing versus a baseline value, the net lead release to blood varied from 0.9 to 10.1 μg/d, which is equivalent to 0.3 to 4.03 mg of lead. With group PbB concentrations usually less than 3 μg/dL, the observed releases imply a high skeletal turnover of greater than 10% and possibly greater than 30% in some subjects during pregnancy and the postpartum period. These elevated rates in some subjects may partly arise from low daily calcium intakes, being one half to two thirds of that of recommended daily requirements. The lead flux calculated from a cumulative approach was compared with other approaches: first-order kinetics, bone turnover, bone x-ray fluorescence measurements, and the International Commission for Radiological Protection lead pharmacokinetic model calculated lead releases and remaining bone lead concentrations would likely not be detectable by current x-ray fluorescence methods.