In recent years a major concern in occupational medicine has been to define the toxicity of lead at lower levels of exposure. Investigations conducted since 1970 have demonstrated that lead causes hematologic, neurologic, and renal impairment in lead workers at exposure levels that previously were considered to be safe. In this report the authors describe three recent epidemiological studies of occupational lead exposure. Their purpose in each instance was to evaluate dose-response relationships between blood lead levels and lead toxicity, particularly at lower levels of occupational lead exposure. In this study evidence was found for clinical, hematological, neurological and renal impairment in lead workers many of whom had blood lead levels between 40 and 80 gu/dl. Such findings suggest that blood lead levels in lead workers should not exceed 40 ug/dl, a value already more than two standard deviations above the mean for adult males in the United States.