Survival in patients with hypoxic chronic obstructive pulmonary disease (COPD) is improved by long-term oxygen therapy (LTOT). Such patients are known to desaturate during activity and at night. The aim of this study was to assess the adequacy of oxygenation in patients with COPD receiving LTOT. Oxygen saturation (SaO2) was measured at home over 24 h in 20 patients with hypoxic COPD receiving LTOT. Eleven had arterial oxygen partial pressure values (PaO2) > or = 8 kPa (Group 1) and nine had PaO2 < 8 kPa (Group 2), at rest, breathing oxygen. There was no difference in SaO2 during the day and night when breathing oxygen. In Group 1, SaO2 was > 90% for 78 +/- 24% of the 24-h period. Four patients spent between 75 and 90%, and three spent < 75% of the 24-h period with SaO2 > 90%. In Group 2, SaO2 was < 90% for 69 +/- 27% of the 24-h period (P = 0.02). Two patients spent between 75 and 90%, and four spent < 75% of the 24-h period with SaO2 > 90%. Measurements of SaO2 over 24 h in patients with hypoxic COPD, while breathing oxygen, add further information to arterial blood gas sampling on the adequacy of oxygenation, and reveal inadequate oxygenation in many patients.