A brief review of the use of stable lead isotope ratio measurements to identify and apportion sources of lead in human tissue is followed by examples of the use of inductively coupled plasma source mass spectrometry for such studies.Inductively coupled plasma source mass spectrometry (ICP-MS) has only recently been used for measurements of coupled ratios in body tissues and fluids and in environmental sources of lead. Generally, the inaccuracy of these measurements is about -0.2% and the imprecision less than 0.5%. This analytical performance is sufficient to detect the much larger changes in(206)Pb:(207)Pb ratios of -2% or higher, seen in environmental lead exposure and in childhood lead poisoning.Measurements of lead isotope ratios by ICP-MS have been used to identify specific sources of childhood lead poisoning and to indicate the relative importance of environmental sources, such as drinking water and lead from petrol. Populations in the United Kingdom with low lead uptake usually have(206):Pb(207)Pb ratios in body tissues within the range 1.13 ± 0.01. Significant deviations from this range have been seen in response to increased uptake from lead in: drinking water in parts of Scotland (source ratio ∼1.18 and petrol lead in inner London (source ratio ∼1.07). The dominant source for some Scottish subjects with high concentrations of lead in blood or in teeth was water, which contributed approximately 60% to body lead. Petrol lead was shown to be a significant contributor (30-40%) to the body lead of inner London children.