To examine the clinical efficacy of a newly developed pressure swing adsorption (PSA) type oxygen concentrator with a membrane humidifier without added water for humidification, the new machine was compared with the conventional PSA type oxygen concentrator with bubble water humidifier in 10 patients with chronic pulmonary disease. Relative humidity, partial pressure of oxygen (PaO2) and partial pressure of carbon dioxide (PaCO2) in arterial blood were measured when the patient breathed air and oxygen from the oxygen concentrators. No significant difference between PaO2 while breathing oxygen flow from the new machine and that while breathing oxygen from the conventional oxygen concentrator was observed. All patients answered that there was no difference on subjective impression between breathing oxygen from the new machine and from the conventional oxygen concentrator. Sufficient relative humidity (above 50%) of oxygen flow was obtained by using membrane humidifier. Since this machine saves the troublesome procedures of cleaning the container and changing the water, it will be beneficial to the patients who use a PSA-type oxygen concentrator in their home.