Since the introduction of bronchoalveolar lavage (BAL), this technique is used both clinically and for research. Use of BAL as a diagnostic aid was introduced fairly recently. For the different steps of the procedure a standardized approach has been proposed. Analysis of BAL fluid includes determination of the total number of cells, of the differentiation and of non-cellular components, as well as various staining and culturing methods. The analysis may be useful in the diagnosis of interstitial pulmonary diseases and opportunistic infections. Smoking and pharmacotherapy affect the results of the analysis. BAL is a safe procedure with hardly any side effects. This method may in some diseases replace more invasive diagnostic procedures such as biopsy by demonstrating characteristic cells or micro-organisms.