1. In four normal subjects and 51 patients with chronic bronchitis-emphysema, bronchial asthma and pulmonary fibrosis, the VD/VT ratio was measured by a helium washout technique (w VD/VT) and by the conventional Bohr's method using the arterial CO2 tension (VD/VT).
2. In 16 patients the w VD/VT ratio was compared with that calculated from Bohr's equation with the alveolar CO2 tension measured by the rebreathing technique (A VD/VT).
3. It was found that the values for the VD/VT ratio obtained by the helium washout technique were very close to those obtained by the conventional method using the Paco2 or P¯Aco2 values.