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Background: The Anesthetic Conserving Device--AnaConDa® (ACD)--has been compared with a conventional vaporizer. However, the accuracy of the administered concentration of volatile anesthetics was not examined. In the present study we measured the accuracy of the ACD when used as a portable vaporizer., Methods: This prospective study included 30 ASA I-III patients scheduled for elective surgery under general anesthesia. The patients were randomly organized into 3 groups of 10 patients per group. In each group, the sevoflurane infusion rate was adjusted to deliver 1.0 vol%, 1.5 vol%, and 2.0 vol% alveolar concentration. Hemodynamic data, bispectral index, and end-tidal sevoflurane concentrations were recorded every 2 minutes., Results: We analyzed 801 data points from 30 patients. The mean difference between the end-tidal sevoflurane concentration and the target concentration was -11.0 ± 9.3% of the target when the target was 1.0 vol%, -5.4 ± 6.4% when the target was 1.5 vol%, and -4.0 ± 7.4% when the target was 2.0 vol%. No significant differences were found in the error at the different target concentrations., Conclusions: We found that the ACD may be a valid alternative to the conventional vaporizer. The ACD is very simple to use, delivery rate needs to be adjusted only once per hour, and the anesthetic savings are independent of the circuit characteristics and fresh gas flow rate.