Potential kidney function decrement with long-term lead exposure is important in the overall assessment of adverse health effects of lead in industrial workers or other exposed groups. Two clinical field studies of secondary lead smelter workers have shown that a significant proportion of workers had slightly to moderately elevated BUN and creatinine levels; the prevalence was higher in those with longer lead exposure. Since a decrement of kidney function with age has been documented, and, since duration of lead exposure may also be strongly related to age, it was necessary to assess the age dependent renal function decrement in a control (non-lead-exposed) population. BUN and creatinine levels in the lead-exposed workers showed a much more significant correlation with age than that which was found in the non-exposed population; the correlations between the indicators of renal function, BUN and creatinine, and duration of lead exposure remained statistically significant after removing the age-dependent decrement derived from the control population. Moreover, a highly significant correlation between BUN and zinc protoporphyrin levels was found. The results indicate a sizeable and significant decrement in kidney function in the secondary lead smelter workers studied; this effect was found to be lead-induced, by removing its age-dependency.