Current evidence related to the carcinogenic potential of ethylene- oxide (75218) (EtO) is presented. Major uses of EtO are in the chemical industry and as a sterilant and fumigant. In its 1977 Special Occupational Hazard Review on EtO, NIOSH recommended that the Occupational Safety and Health Administration's (OSHA) 8 hour time weighted average exposure level of 50 parts per million (ppm) be observed, and that occupational exposure be limited to a ceiling concentration of 75ppm. Studies in Fischer-344-rats exposed by inhalation to 10, 33, or 100ppm EtO for 6 hours/day, 5 days/week, for about 2 years showed dose related excesses of mononuclear cell leukemia in females and peritoneal mesothelioma in males, both of which were significantly increased. Preliminary evidence of dose dependent injection site sarcomas after chronic subcutaneous injection of EtO were obtained in NMRI-mice. In-vitro microbial and animal test systems showed mutagenicity and alkylating activity of EtO, and mutagenic and adverse reproductive effects and chromosomal abnormalities were found in experimental animals. Epidemiologic studies of exposed worker populations demonstrated possible carcinogenic activity, with increases in overall death rates, all cancers, leukemia, stomach cancer, and circulatory system mortality. Exposures were generally below current standards, although occasional excursions to higher levels were possible. Chromosome aberrations and sister chromatid exchanges in peripheral lymphocytes were increased in one exposed worker population studied. It is recommended that ethylene-oxide be considered a potential occupational carcinogen and that the OSHA standard be reexamined. Suggested measures for minimizing worker exposure include product substitution, contaminant controls, worker isolation, and use of personal protective equipment.