OBJECTIVES: To evaluate the impact of The Danish Oxygen Register on COPD patients' treatment modalities, survival, and adherence to guidelines for long-term oxygen therapy (LTOT). DESIGN: The Danish Oxygen Register. SUBJECTS: 8487 COPD patients who received LTOT in the study period from November 1, 1994, to December 31, 2000. MAIN OUTCOME MEASURES: Follow-up, smoking status, correct prescription of LTOT (15-24h/day), and survival. RESULTS: During the study period an increasing number of patients were prescribed LTOT in connection with discharge after hospitalisation for an exacerbation (1995 vs. 2000: 74.4% vs. 82.2%, P<0.001), were prescribed oxygen 15-24h/day (66.2% vs. 85.5%, P<0.001), had delivered oxygen concentrator or liquid oxygen (77.8% vs. 96.9%, P<0.001), and had mobile oxygen (29.9% vs. 42.8%, P<0.001). Only 65.8% of the patients were followed-up in an outpatient clinic with the possibility of re-evaluation of the criteria for LTOT and adjustment for oxygen flow, with no change during the study period (P=0.43). In a representative subsample, 77.1% had smoking habits or measurement of CO-level registered in 1995 compared to 79.6% in year 2000 (P=0.65), and 25.1% vs. 21.2% (P=0.34) were considered current smokers. The median survival increased from 1.07 to 1.40 years (P=0.032). CONCLUSIONS: Adherence to guidelines for LTOT has improved concerning administration of oxygen, but has remained poor concerning follow-up of the patients and smoking cessation. Survival of COPD patients on LTOT has improved during the observation period.