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 blood urea nitrogen (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 control (non-lead-exposed) populations. 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 populations; the correlation remained statistically significant after correcting for the age-dependent decrement derived from the control populations. Moreover, highly significant correlations between BUN and creatinine and the biological indicator of lead absorption, zinc protoporphyrin, were found. The results indicate a sizeable and significant decrement in kidney function in the secondary lead smelter workers studied; by removing its age-dependency, this effect was found to be lead-induced. In other occupational groups, with lesser and/or shorter lead exposure, a significant effect on renal function as reflected in BUN and creatinine levels could not be detected.