A long-term (14-18 yr) prognostic study on patients with severe Chronic Airflow Obstruction (CAO) (FEV1 less than 1000 ml) is reported. In 65 of the 79 patients under study at least 7 serial FEV1 values were available. At the beginning of the study long-term treatment with oral prednisolone in doses of 10-15 mg/d was started. Side-effects and introduction of inhalation corticosteroid therapy resulted in a decrease or complete cessation of oral corticosteroid treatment. Three distinct patterns of the course of FEV1 were recognized: 1) no change, 2) initial increase followed by decrease, and 3) linear decrease. Initially 138 clinical parameters, including reversibility of airflow obstruction, were comparable in groups 1, 2 and 3; group 3 showed somewhat stronger evidence of emphysema in lung function parameters. The 3 patterns of FEV1 showed strong association with the long-term use of prednisolone. At an oral dose of 7.5 mg or less/d, FEV1 decreased, often after a considerable time-lag (6-32 months). The results of the study suggest that in CAO oral prednisolone, in doses above 7.5 mg/d, may slow down progression of the disease.