Pulmonary emphysema is a pathologic diagnosis. However, the diagnosis can be made with relative certainty on the basis of clinical and radiologic criteria. The clinical diagnosis of emphysema can be difficult because correlations between results of lung function tests and the extent of emphysema are poor. Additionally, features of other obstructive lung diseases may overlap. From the clinician's standpoint, the main value of differentiating between emphysema and other obstructive diseases of the airways (e.g., asthma and chronic bronchitis) is to (1) establish a prognosis and (2) guide the use of corticosteroid therapy by defining the degree of reversibility that can be expected in patients with limitation in air flow.