The comments on my article in CHEST (September 2008)1 about the diffusing capacity of the lung for carbon monoxide (Dlco) by Drs. Frank and Johnson are appreciated. Both point out that the alveolar volume (VA) measurement used to calculate the Dlco/VA ratio tends to underestimate VA, thereby falsely elevating the ratio. Dr. Frank suggests that one way to avoid this is to use a calculation consisting of a separately measured total lung capacity minus the anatomic dead space to substitute for the VA measurement. This maneuver helps him to interpret the ratio. Dr. Johnson has developed equations from his study2 on the effect of lung volume changes on the Dlco in 24 healthy subjects that appear to improve the volume correction for the Dlco. He also advocates that the Dlco/VA ratio should be volume corrected as well. These adjustments improve his ability to interpret diffusion. Further studies on the use of Dr. Johnson's equations would be helpful to determine whether these adjustments of the Dlco, VA, and Dlco/VA ratio would improve their clinical utility.