An increased incidence of sister chromatid exchanges (SCEs) in peripheral lymphocytes of operating room personnel exposed to waste anesthetic gases has been reported. We investigated whether the increase of SCEs in anesthesiologists was reversible. Twenty-five anesthesiologists exposed to waste anesthetic gases such as sevoflurane and nitrous oxide were compared with nonexposed internists working in the same hospital. The concentrations of sevoflurane and nitrous oxide in the operating rooms were measured. The incidence of SCE was measured in lymphocytes cultures of anesthesiologists before and after a 2-mo leave from the operating room. These values of SCE were compared with those of nonexposed physicians. Occupational exposure to sevoflurane and nitrous oxide in the operating rooms were above the threshold values. There was a significant difference in SCE values of the anesthesiologists compared with the nonexposed physicians (11.9 +/- 4.4 versus 4.2 +/- 1.1, P < 0.001). After a 2-mo leave from the operating room, the SCE values of the anesthesiologists were significantly lower compared with those taken before the leave (4.8 +/- 1.8 and 11.9 +/- 4.4, respectively, P < 0.001). We conclude that the increase of SCE in anesthesiologists exposed to increased environmental concentrations of waste anesthetics gases, such as sevoflurane and nitrous oxide, are reversible if they work free from exposure for 2 mo.