Biological monitoring of exposure to ethylene oxide (EO) was carried out in 31 hospital workers in the sterilization facilities of five hospitals. The excretion of total thioethers was determined in the urine of sterilization workers. An occupational non-exposed group served as a control (n = 31). The air EO level was not monitored routinely, however, peak concentrations of EO up to 200 p.p.m. were detected mainly when the sterilization chambers were open. Our results show a significant difference in urinary thioether excretion between the exposed and control groups (P<0.001). Sterilization workers seem to absorb significant quantities of EO. We feel that other alternative and more sensitive methods for detecting thioether metabolites of EO should be carried out to confirm our results.