In discussing the diffusing capacity of the lung for carbon monoxide (Dlco), Dr. Plummer1 points out the difficulty in “normalizing” the value with the alveolar volume (VA) because the relation between the two is nonlinear. There is another difficulty with the Dlco/VA ratio. Lung volumes determined by plethysmography or nitrogen washout are usually done along with the diffusing capacity. Frequently, I have observed that the VA derived from helium or methane dilution is considerably smaller (ie, upwards of 1 L) than the total lung capacity derived from plethysmography, even in patients whose dead space volume would be expected to be normal. In such patients, the VA should equal the total lung capacity minus an assumed dead space volume (roughly 2 mL/kg). In patients whose Dlco is low, the use of a VA derived from dilution may result in a “normal” Dlco/VA. In such cases, I calculate Dlco/VA using the total lung capacity and an assumed dead space volume, to see whether this confirms a normal Dlco/VA, which may guide interpretation of the test.