Bronchoalveolar lavage (BAL) is a relatively new technique for assessment of the alveolar compartment of the lung. Its current clinical applications are the initial assessment of acute and chronic diffuse interstitial lung disease and the diagnosis of pulmonary infections, especially in the immunocompromised host. In addition, BAL provides information on the pathophysiology of diffuse interstitial lung disease. Future studies of BAL fluids should confirm its use in the prognostic assessment of diffuse interstitial lung diseases and in the assessment of effects of therapy.