This chapter provides an overview of available information concerning chronic health effects in human populations exposed to ethylene oxide (EO). Evidence of carcinogenicity is available from a 2-year chronic inhalation study which linked EO to the induction of peritoneal mesotheliomas in male rats and mononuclear cell leukemia in female rats. The suggestion has been made that acute high level exposures to EO may stimulate total leukocyte or lymphocyte production. Sister chromatid exchanges (SCEs) have been examined in studies of exposed workers. The question of the relationship of chromosomal aberrations to SCEs is of considerable interest. The frequencies of induced SCEs underwent an exponential decline with holding time, whereas the mutation frequencies remained constant. The conclusion is reached that both the quantity and quality of the available epidemiology studies are inadequate to permit any sound decisions to be made regarding quantitative health risks for exposed workers.