Daily duration of oxygen administration is an important factor in the effectiveness of long-term oxygen therapy (LTOT) for hypoxic chronic pulmonary disease. We have assessed the daily use of oxygen therapy in 930 patients with COPD and examined factors associated with the effective use of this treatment.Objective daily duration of oxygen use over a 3-month period was prospectively measured using the counter clock of the oxygen concentrators or by weighing the liquid oxygen container at each delivery. A questionnaire was filled in by an independent investigator asking about home situation, lifestyle, and whether oxygen therapy was used during all domestic and outside activities. In addition, prescribing physicians were asked about the duration and modalities of oxygen prescribed in each case.The patients had been receiving LTOT for 36 +/- 24 months and had hypoxemia (PaO2 = 56 +/- 9 mm Hg), hypercapnia (PaCO2 = 47 +/- 8 mm Hg), and severe airflow obstruction (FEV1/VC = 42 +/- 14%). The mean duration of oxygen treatment prescribed was 16 +/- 3 h/d. The mean duration of oxygen therapy achieved was 14.5 +/- 5 h, but only 45% of the patients achieved oxygen therapy for 15 h or more per day. Patients with effective use of LTOT, ie receiving oxygen therapy for at least 15 h/d, were significantly more hypoxic (PaO2 = 54 +/- 9 vs 57 +/- 9 mm Hg; p < 0.001), more hypercapnic (PaCO2 = 48 +/- 8 vs 46 +/- 7 mm Hg; p < 0.005), and also more obstructed (FEV1/VC = 39.5 +/- 13 vs 45 +/- 14%; p < 0.001) than the rest of the patients under treatment. Other factors associated with effective use of oxygen were (1) initial prescription for 15 h or more per day, (2) supplementary education on oxygen therapy by a nurse or physiotherapist, (3) cessation of smoking, (4) use of oxygen in all domestic situations (toilet, meals, leisure, etc.), and (5) absence of side effects from oxygen treatment.Attention by the prescribing physicians to such factors could optimize oxygen prescription and constitute goals for education of patients.