Pulmonary function tests were performed in 18 cases of obstructive sleep apnea in supine and sitting positions and the relationship between pulmonary function and polysomnographic data was analyzed. %FRC and PaO2 were reduced in the supine position compared with those in the sitting position. It was suggested that the reduction of PaO2 was mainly caused by the elevation of CC/FRC ratio in supine position. The relationship between pulmonary function data and polysomnographic data were analyzed, and an inverse relationship between %FRC in sitting position and desaturation was observed and also a positive relationship between PaO2 in the supine position mean-nadir SO2 was found. Saw-tooth sign and VE50/VI50 greater than 1 on flow-volume curves were not related to the apnea index and desaturation index. These results indicate that the F-V curve is not useful for the diagnosis of OSA.