The safety of low-flow sevoflurane anesthesia, which produces higher concentrations of toxic compounds, has been questioned. One hundred surgical patients received sevoflurane or isoflurane anesthesia at a total flow rate of 1 L/min. End-tidal CO2 concentrations and inspired and end-tidal anesthetic concentrations were monitored during anesthesia. Pre- and postanesthetic clinical laboratory studies were performed in both groups, and no significant differences were found between groups. In the sevoflurane group, the concentrations of degradation products in the circuit were measured by gas chromatography and the temperature of the CO2 absorbent was also measured. Two degradation products were detected: CF2 = C(CF3-O-CH2F (Compound A) and CH3OCF2CH(CF3)OCH2F (Compound B). The highest measured mean concentration of Compound A was 24.6 +/- 7.2 (13.6-41.3) ppm, and that of Compound B (detected in 12 patients) was 1.5 ppm. In both groups, total bilirubin, direct bilirubin, aspartate aminotransferase, and alanine aminotransferase were increased postoperatively. There was no difference between groups. Low concentrations of Compound A were present in low-flow sevoflurane anesthesia, but no significant differences in clinical laboratory values were observed between low-flow sevoflurane and isoflurane anesthesia.