Background: For patients undergoing general anesthesia, adequate warming and humidification of the inspired gases is very important. The aim of this study was to evaluate the differences in the heat and moisture content of the inspired gases with low-flow anesthesia using 4 different anesthesia machines. Methods: The patients were divided into 11 groups according to the anesthesia machine used (Ohmeda, Excel; Avance; Dräger, Cato; and Primus) and the fresh gas flow (FGF) rate (0.5, 1, and 4L/min). The temperature and absolute humidity of the inspired gas in the inspiratory limbs were measured at 5, 10, 15, 30, 45, 60, 75, 90, 105, and 120minutes in 9 patients scheduled for total thyroidectomy or cervical spine operation in each group. Results: The anesthesia machines of Excel, Avance, Cato, and Primus did not show statistically significant changes in the inspired gas temperatures over time within each group with various FGFs. They, however, showed statistically significant changes in the absolute humidity of the inspired gas over timewithin each groupwith low FGF anesthesia (P< .05). The anesthesia machines of Cato and Primus showed statistically significant changes in the absolute humidity of the inspired gas over time within each group with an FGF of 4L/min (P< .05). However, even with low-flow anesthesia, the temperatures and absolute humidities of the inspired gas for all anesthesia machines were lower than the recommended values. Conclusion: There were statistical differences in the provision of humidity among different anesthesia workstations. The Cato and Primus workstations were superior to Excel and Avance. However, even these were unsatisfactory in humans. Therefore, additional devices that provide inspired gases with adequate heat and humidity are needed for those undergoing general anesthetic procedures. Abbreviations: FGF = fresh gas flow, IGT = Inspired gas temperature, AH = Absolute humidity.