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We performed this study to determine the effect of heated wire circuits on humidity delivered by several humidifiers commonly used during anesthesia. Humidifier bases from three manufacturers with several humidifying chambers for each base were tested using both heated and unheated circuits. Delivered humidity and circuit temperature were measured at the distal end of the circuit. Each chamber was tested under both continuous flow (CF) and intermittent flow (IF) conditions. IF was provided by a Servo 900 B ventilator. Adjustments of flow and volume were made for pediatric chambers. At 90 L/m CF, humidifier performance was unsatisfactory with both heated and unheated circuits. At 15 and 45 L/m CF, both distal circuit temperature (EXIT) and humidity were within acceptable limits; unheated circuits delivered more humidity than heated circuits (P < 0.05). Relative humidity was lower at 45 L/m CF than at 15 L/m (P < 0.05). During IF while all chambers provided satisfactory humidity, humidity was greater when unheated circuits were used. We conclude that when heated circuits are used, humidifying systems may fail to deliver 100% humidity to patients.