In 16 patients with chronic bronchitis and advanced cor pulmonale admitted to hospital due to heart failure, a controlled 6-weeks oxygen therapy, 17 h a day, was performed. The trial was started when patients were out of the exacerbation of the disease in a respiratory and circulatory steady state. Pulmonary hemodynamics, blood viscosity, packed cell volume and basic lung function data before and after oxygen therapy were compared. A significant fall of mean pulmonary arterial pressure from 42 to 30 mm Hg without a change in the cardiac output was found. Blood viscosity and packed cell volume-significantly decreased. Arterial oxygen tension did not significantly change. It seems that 17-hours per day oxygen therapy is sufficient to induce regression of anatomic changes in pulmonary arteries, a main cause of pulmonary hypertension in hypoxic cor pulmonale.