A group of workers occupationally exposed to lead have had measurements of their tibia lead concentrations made on two occasions separated by five years; on the second occasion calcaneus lead concentrations were also measured. The results serve to confirm the reliability of the measurement technique and to illustrate the improved precision achieved through technical improvements. More importantly, the relation between tibia lead concentration and cumulative blood lead found in this longitudinal study was entirely consistent with that previously reported, which had been based on cross sectional studies. Furthermore, the relation between lead concentrations in the tibia and in calcaneus found here was similar to that previously found in a larger cross sectional survey. It is concluded that this technique of measuring bone lead concentrations non-invasively is likely to be used increasingly as a biological monitor of cumulative exposure to lead.