ObjectiveTo investigate: a) the relationship between humidifier temperature and inspired gas humidity and b) the effect of insulating the inspiratory tube on “rainout” (condensate).DesignObservational study.SettingRegional neonatal unit in a university hospital.PatientsForty-eight infants receiving assisted ventilation, of whom 31 infants were nursed in incubators and 17 under radiant heaters.Measurements and Main ResultsDespite always maintaining humidifier temperature <34.7°C, inspired gas humidity decreased below the American National Standards Institution minimum of 30 mg H2O/L on 35 of 479 occasions. At a humidifier temperature of 36°C, inspired gas humidity varied between 17 and 43 mg H2O/L. In incubators set at a temperature of 34.1 ± 1.3 (SD) °C, inspired gas humidity was linearly related to humidifier temperature, but with wide scatter (p< .001, r2= .28). In cooler incubators set at 32.9 ± 1.8°C, inspired gas humidity varied inversely with humidifier temperature. This variation was attributed to condensate due to inspired gas cooling within the incubator. Insulation of the inspiratory tubing reduced condensate by only 15%.ConclusionsInspired gas humidity cannot be predicted reliably from humidifier temperature. Accurate control will require a new generation of humidifiers that measure inspired gas humidity.