Ethylene oxide was tested in environmental air and in the alveolar air of 10 workers employed in a hospital sterilizer unit at hourly intervals during the work shift. Alveolar ethylene oxide concentrations (Ca) were correlated with environmental concentration (Ci) in all the workers studied (r = 0.89-0.99). The ratio between alveolar and environmental concentration (Ca/Ci) given by the slope of the regression line obtained for all the data collected was 0.24. This means that the alveolar retention of ethylene oxide, expressed as 1 - (Ca/Ci), corresponded, on average, to about 75% of the environmental concentration.