The relationship between respiratory insufficiency, expressed by gas tensions in blood and bone marrow, and haematologic adaptation has been studied in 82 men with chronic bronchitis and persistent breathlessness for at least one year. For the group as a whole and for various subgroups, a linear correlation was demonstrated between arterial hypoxia and Hb concentration, haematocrit, blood volume, Hb mass and erythrocyte volume, respectively. The increase in Hb concentration compensated for the fall in arterial oxygen saturation. Only for one subgroup, with a pH difference between arterial and venous blood larger than or equal to 0.08, was no correlation found between the degree of hypoxia and Hb concentration. The haematologic parameters correlated significantly better with arterial oxygen tensions than with oxygen tensions in the bone marrow. There was no indication that decreased oxygen supply to the bone marrow led to the increased erythrocyte and Hb production in patients with arterial hypoxia caused by chronic bronchitis.