Bronchoalveolar lavage is an invaluable means of accurately evaluating the inflammatory and immune processes of the human lung. Although lavage recovers only those cells and proteins present on the epithelial surface of the lower respiratory tract, comparison with open lung biopsies shows that these constituents are representative of the inflammatory and immune systems of the alveolar structures. With the use of these techniques, sufficient materials are obtained from normal individuals to allow characterization of not only the types of cells and proteins present but their functions as well. Such observations have been useful in defining the inflammatory and immune capabilities of the normal lung and provide a basis for the study of lung disease. Lavage methods have been used to characterize inflammatory and immune processes of the lower respiratory tract in destructive, infectious, neoplastic, and interstitial disorders. From the data already acquired, it is apparent that bronchoalveolar lavage will yield major insights into the pathogenesis, staging, and therapy decisions involved in these disorders. (Am J Pathol 97:149--206, 1979).