Destruction and loss of lung tissue is the hallmark of the disease we call emphysema. Emphysema is primarily a disease of lung tissue. It is classifi ed as a disease causing chronic airfl ow obstruction or COPD because within the lung tissues are small elastic fi bers which attach to the small airways and help to keep them open. Usually, patients with emphysema demonstrate reduced airflow on pulmonary function tests. I was reminded that this reduction in airflow does not always parallel the severity of emphysema, and the reduction in airfl ow seen on pulmonary function studies in these patients does not always correlate with symptoms. I was seeing a patient who had been complaining of severe shortness of breath on exertion, but his pulmonary function studies showed only a modest degree of airfl ow obstruction. Fortunately for this patient, he also had a diffusing capacity study done along with a CT scan of the chest. A diffusion study is part of our complete and more sophisticated pulmonary function test. A simple spirometry measures only air fl ow. Diffusion studies tell us about the function of the lung tissue itself. The diffusion study in this patient was severely reduced and his CT scan showed marked tissue destruction of the lung by emphysema. The diagnosis in this patient was made because of more sophisticated pulmonary function testing and CT scanning. The search for the cause of this patient’s symptoms was over. Sadly, no therapy can restore his damaged and absent lung tissue, but some help is available.