In this paper I briefly review the significance of the single breath test of diffusing capacity for carbon monoxide (DLCO). I then attempt to answer the question of why this simple test has not become widely used in the past twenty years. The reasons are primarily due to failure in communication between research workers and practicing physicians. The practicing clinicians have difficulty with mathematical analyses and do not appreciate the limits for which the test is valid. Under steady-state conditions DLCO measures the transfer factor for the whole lungs, i.e. the volume of CO taken up by pulmonary capillary blood from alveolar gas when the alveolar CO pressure is one. It is independent of regional inhomogeneities. DL is the product of Krogh's K and alveolar volume. Krogh's K tends to become constant near total lung capacity while DL does not. For this reason K will probably vary less than DL and should always be reported when doing this test. In terms of the simple lung model with a single homogeneous alveolar compartment K is equal to DL per unit alveolar volume. I expect the measurement of K and DLCO by the modified Krogh single breath test to become a screening test for lung disease in its early stages.