Considerable progress has been made in understanding the similarities and differences among the forms of the interstitial lung diseases (ILDs), particularly the forms of idiopathic interstitial pneumonia, now recognized as distinct clinicopathologic entities. Lung parenchymal evaluation by high-resolution CT scanning of the chest may provide images that are virtually diagnostic of certain forms of ILD, but other testing, including bronchoalveolar lavage (BAL) and lung biopsy, may be required for accurate diagnosis. The differential diagnosis of these disorders rests on the clinician's interpretation of the clinical presentation and physical examination findings, pulmonary function testing, radiographic imaging, and, if required, sampling of lung tissue. This discussion examines the usefulness of BAL in the diagnosis of specific forms of ILD.