Bronchoalveolar lavage (BAL) was performed in 1,188 patients suffering from sarcoidosis. After technical considerations, the authors analyze the results of BAL from a practical point of view concerning its value for the diagnosis of sarcoidosis and its prognostic value and its value for the selection of therapy, particularly for the decision as to steroid treatment. BAL helps in the diagnosis of sarcoidosis, but is not specific enough to provide this diagnosis on its own. The persistence of high alveolar lymphocytosis within the first year of evolution is strongly correlated with nonrecovery from pulmonary sarcoidosis at 2 years and thus with the evolution towards a chronic phase of the disease. On the other hand, BAL can provide basic information for a better understanding of the disease and permits immunocompetent cells and soluble factors to be recovered from the lung, which is useful for immunological studies.