The Office of Scientific Affairs within the California Department of Toxic Substances Control has reviewed the evidence on the carcinogenicity of crystalline silica. The authors consider the current evidence to be convincing to classify crystalline silica as a human carcinogen by the inhalation route. The weight of evidence for both rats and humans indicates that fibrotic and silicotic lesions in the lung result from inhalation exposure to crystalline silica and that lung cancer is secondary to those lesions in the lung. Thus crystalline silica should be considered to have a threshold for causing cancer. The critical exposure criterion is that exposure level which does not produce a fibrogenic or silicotic response; thus it is necessary to determine the no observed adverse effect level (NOAEL) for fibrogenesis. The authors recommend that the United States Environmental Protection Agency review all appropriate studies to develop a reference concentration that can be used for regulatory purposes.