Condensation in the external airways of patient ventilation systems using conventional warm-water humidifiers increases the risk of bacterial contamination. Condensate formation can be reduced by heating the external airway and reducing the length of tubing between the patient and humidifier. The method described incorporates the humidifying element within the interconnecting tubing itself. Air is supplied through two semi-permeable tubes, of polytetrafluoroethylene, which are encased in an outer shell. A warm-water jacket is maintained in the shell; this is the source of heat and water vapour which diffuses through the semi-permeable tube wall and into the patient's air supply. An evaluation of performance and an initial clinical trial are reported.