The toxic hazards of exposure to chloromethyl-methyl-ether (3188134) (CMME) are discussed. The industrial uses of CMME in the manufacture of ion exchange resins and polymers, in the treatment of textiles, and as a solvent for polymerization reactions are noted. The presence of bis(chloromethyl)ether (542881) (BCME) as a contaminant in CMME is described in relation to assessing the hazards of CMME exposure. The results of various studies of CMME carcinogenicity in laboratory animals are reviewed, including injection, skin application, and inhalation tests in rats and mice with induction of papillomata, squamous cell carcinomas, fibrosarcomas, lung adenomas, and lymphosarcomas. Also discussed are several epidemiological studies of workers exposed to CMME in which increased incidences of lung cancer, particularly oat cell carcinoma, were found. The possibility is considered that these increased lung cancer rates were due to concomitant BCME exposure, rather than to CMME exposure. The difficulty in obtaining or maintaining pure CMME is noted due to its rapid decomposition into reactants that produce BCME. The author concludes that CMME should be considered an occupational carcinogen, even though the unavoidable concomitant exposure to BCME may ultimately preclude the definitive determination of CMME carcinogenicity.