In a group of 80 male patients (average age 52.5 years) with severe obstructive ventilatory impairment (FEV_{1 0} less than 40% of predicted) due to chronic bronchitis and/or emphysema, the blood carbon dioxide tension measured at rest, in a stable clinical status, was confronted to the spirometric variables and the oxygen tension. PaCO₂ (range 31–60 mm Hg) was loosely related to the VC (r = -0.28) and the FEV (r = -0.30, both p < 0.05) and strongly related to the PaO₂ (r = 0.60; p < 0.001). When patients were classified as bronchitic or emphysematous according to clinical, roentgenologic and biological criteria, the correlations above were found to be higher for bronchitics. In patients with chronic bronchitis with severe obstruction a very good estimation of PaCO₂ is possible from PaO₂: PaCO₂ = 75.8–0.44 PaO₂ (SEE 0.1 mm Hg).