Long-term domiciliary oxygen therapy in patients with chronic respiratory failure significantly improves both survival and quality of life. These therapeutic objetives are only achieved by daily oxygen therapy of more than 15 hours. For a period of 3 months, we have prospectively measured the duration of oxygen therapy in 930 patients with chronic airflow obstruction. This is carried out by reading the meters on the oxygen concentrators, or for liquid oxygen by checking the weight of the cylinders at each delivery, making allowances for the flow rate and also for natural loss from evaporation. The instructions for oxygen therapy and the true therapy of the patient were then gathered using a questionnaire. The practitioners were questioned on the prescription for oxygen therapy which had been made for each patient, and more generally on their usual criteria for prescribing long-term oxygen therapy. The patients (82% male) were aged between 67 +/- 8 years, and were on domiciliary oxygen therapy 36 +/- 24 months, with hypoxaemia (PaO2 = 56 +/- 9 mmHg), hypercapnea (PaCO = 47 +/- 8 mmHG) and suffering from airflow obstruction (FEV1/VC = 42 +/- 14%). The duration of prescribed oxygen therapy was on average 16 +/- 3 hours. The mean duration of oxygen therapy achieved was 14.5 +/- 5 hours, but only 45% of the patients (419/930) managed daily oxygen therapy superior of equal to 15 hours and were categorised as compliant.(ABSTRACT TRUNCATED AT 250 WORDS)