The usefulness of the transcutaneous oxygen tension (tcPO2) in adults is under controversy. In a varied group of respiratory patients, results of the application of this method were compared with those from the arterial blood sampling method. Thirty-eight arterial oxygen tension (PaO2) and tcPO2 simultaneous determinations were made in a group of 22 patients, while in a sitting position; the tcPO2 measurements obtained (68 +/- 12.36 Torr) were significantly lower (p less than 0.05) than the PaO2 values (74 +/- 13.07 Torr). The correlation coefficient was 0.51 (p less than 0.01) with a regression line, tcPO2 = 31.58 + 0.48 PaO2. It is concluded that tcPO2 measurement does not correlate well with PaO2 and that this method cannot be always be safely applied and used in adults with respiratory diseases.