Ethylene oxide, a gaseous sterilant extensively used within health care facilities, is a mutagen in bacteria and in human lymphocytes. The Environmental Protection Agency as well as the National Institute of Occupational Safety and Health have recently stipulated certain conditions for the use of ethylene oxide despite the lack of case reports or epidemiologic studies concerning carcinogenicity. Three cases of leukemia that occurred between 1972 and 1977 in a relatively small group of Swedish workers exposed to ethylene oxide are reported. According to national statistics, 0.2 cases of leukemia would have been expected. The time-weighted average ethylene oxide concentration was 20 .+-. 10 ppm.