The test for the diffusing capacity of the lung for carbon monoxide (DLCO) has been available for nearly 100 years for research and clinical purposes. The single-breath method is used almost exclusively in the United States It has been available in clinical pulmonary function laboratories for > 50 years. DLCO has great value in evaluating patients with lung diseases. Guidelines to standardize DLCO have been published by the American Thoracic Society and European Respiratory Society to reduce the interlaboratory variability that has existed. One code, 94720, should be reported for the billing for DLCO. Another code, 94725, the membrane diffusing capacity, exists for the measurement of the membrane and blood components of the DLCO. Currently, no clinical indications exist for the use of the membrane diffusing capacity. The finding that the number of tests in the Medicare population coded with 94725 has increased by > 1,000% from 2000 to 2005 is quite surprising. This rate is 14-times higher than the rate of increase in the utilization of 94720 over the same period. The possible reasons for these increases are discussed, but the most likely explanation is the financial gain derived from coding 94725. It is proposed that coding and billing of 94725 be stopped until the clinical indications for membrane diffusing capacity have been established. Those who code and bill for 94725 must be prepared to justify the use of this code to Medicare and third-party payers.