A follow-up study of 93 patients with CNSLD over a period of 7.5-10 years disclosed a death rate of 18.3 % (17 patients). Patients with a largely reversible bronchial obstruction (paroxysmal bronchial asthma) proved to show the lowest death rate. 4 (10.8 %) died, of whom only 2 (5.4 %) as a result of their illness. It proved possible to divide the patients with irreversible bronchial obstruction into 2 categories: a group with normal blood gas values at rest and during exercise, and a group with chronic hypoxia. In the former group, 3 patients died (13.6 %), of whom 2 (9.1%) as a result of their illness. In the latter group there were 10 deaths (38.5 %), of which 9 (34.6 %) were due to the disease. Factors determining the prognosis were identified as: chronic hypoxia and especially chronic hypercapnia, ECG changes and signs of decompensation of the right heart. Less important factors in this respect are ventilatory parameters of pulmonary function (VC, FEV1, RV and unequal ventilation). Measurements of the carbon monoxide diffusing capacity by techniques ensuring optimal independence of unequal ventilation showed that the diffusing capacity has no effect on the prognosis.