The persistence of smoking habits results in a worse prognosis of patients with chronic airflow limitation. Some authors consider that oxygen therapy at home (OTH) is meaningless if the patient does not give up smoking. To assess the number of smokers in a group of 65 patients on OTH, a questionnaire on smoking habits was administered and the CO concentration in the expired air was measured. 27.7% of the study patients smoked; 12.3% admitted that they smoked on being questioned and the remaining 15.4% were identified by CO concentrations in the expired air equal to or higher than 10 ppm. There were no significant differences between the groups of smokers and nonsmokers regarding age and pulmonary function, but the rate of those declaring a poor compliance with OTH was higher among smokers.