Initial studies of in-use contamination of mechanical ventilator circuits with humidifiers have indicated that neither the rate of bacterial contamination of inspiratory-phase gas nor the incidence of pneumonia was significantly increased when tubing was changed every 24 hours rather than every 8 or 16 hours. In addition, the incidence of nosocomial pneumonia was not significantly higher when circuits were changed every 48 hours rather than every 24 hours . These findings indicate that the recommended daily change in ventilator circuits may be extended to greater than or equal to 48 hours.