BACKGROUND: Patients afflicted with COPD and respiratory Failure often experience a decrease of oxygen saturation (SaO (2)) under physical strain. The aim of our study was to find out which changes occur in walking distance and SaO (2) under the application of Oxygen (O (2)) 2 l/min continous flow, under demand (D) and under normal air (RL) when performing the Six-Minute-Walking-Test (6MWT) and further, in which way do the 10 patients of the shortest walking distance group (group I) differ from the eo patients with the longest walking distance (group II) concerning the starter 6 MWT (under CF). METHOD AND PATIENTS: 27 patients undertook a 6MWT three days in a row with CF, D and (RL), 6 minutes before (phase I), during (phase II) and after (phase III) the 6MWT oxygen was applied in CF (day I), D (day II) and RL (day III), the mean oxygen saturation in each phase was measured and the change (DeltaSaO (2)) during and after the 6MWT was calculated. Additionally we measured the DeltaSaO (2) between start and middle, as well as between middle and end of each phase. In close proximity to the tests FEV (1) and pO (2) was determined. RESULTS: In the total of all test persons there was no significant difference in the walking distance tetween the 3 forms of application. There was, however, a highly significant decrease of the oxygen saturation. The decrease in the two forms of oxygen application did not differ significantly. The decrease of SaO (2) under strain and the consecutive rise under rest occurred within the first 3 minutes of each phase. Patients of group II were often able to increase their walking distance under RL, whereas patients of group I decreased their walking distance further. In group II FEV (1) abs. was significantly higher, but FEV (1) % debit and pO (2) did not differ significantly between both groups. CONCLUSIONS: Patients with a long walking distance showed a marked learning effect even under RL, they presented a higher FEV (1), but not an increased pO (2). The phase of resaturation after the end of strain is short. Oxygen application through a demand valve with a flow rate of 2 l/min is not inferior to a continuous flow. In more than half of all patients the SaO (2) fell below 90 % under both forms of oxygen application, in conclusion the flow rate should be increased in these cases.