The blood volume and arterial blood gases of 18 patients with chronic obstructive lung disease were studied over a period of 1/2 to 5 years, including 35 acute exacerbations of the lung disease. Treatment during exacerbation was directed at infection, bronchial obstruction and hypervolemia. Long-term diuretic therapy was instituted during the follow-up period. On admission all patients suffered from severe dyspnoea, all but two had signs of peripheral oedema and/or liver congestion and one-third had increased jugular venous pressure. Blood volume was increased in all patients and eight of them had a hematocrit above 50%. PaCO2 was severely reduced and PaCO2 increased on admission. At discharge, these symptoms and signs had all diminished or disappeared. Blood volume had fallen an average of 11 and a further reduction of 0.41 was noticed during the follow-up period. Blood gases had improved by discharge and a further improvement accompanied the reduction of blood volume during the follow-up period. It is suggested that 1) hypervolemia is common in patients with advanced chronic obstructive lung disease; 2) hypervolemia may impair arterial oxygenation; 3) long-term diuretic therapy seems to be necessary for maintaining a normal blood volume.