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Bronchioloalveolar lavage (BAL) enables diffuse interstitial lung disease to be divided into lymphocytic and granulocytic alveolitis. The combination of BAL and transbronchial lung biopsies using modern flexible fiberoptic bronchoscopes allows the subdivision of lymphocytic alveolitis into sarcoidosis, exogenic allergic alveolitis (synonym: hypersensitivity pneumonitis: EAA) and granulomatous pneumonias caused by infectious agents. The use of immunohistochemical surface markers of lymphocytes in conjunction with BAL provides further differentiation of lymphocytes into T- and B-, T-helper and T-suppressor types, natural killer cells (NK cells) and cytotoxic T-cells. A predominance of T-suppressor lymphocytes is an indication of EAA, whereas a predominance of T-helper lymphocytes is positively correlated with sarcoidosis. Other markers, e.g. HLA-DR, when expressed on the surface of alveolar macrophages, merely indicate activation unrelated to a specific type of lymphocytic alveolitis. BAL is also a new and promising diagnostic tool for pneumoconioses and other types of lung disease caused by inhaled industrial pollutants. Ferruginous bodies and silica crystals, free or ingested by alveolar macrophages, can be found more easily than by scraping tissue blocks or from multiple sections of transbronchial biopsies. BAL cells can easily be processed for electron microscopy and inhaled foreign material can be analysed in an electron microscope using X-ray diffraction analysis (EDAX) or electron spectroscopic imaging (ESI). BAL is also of value in the diagnosis of peripheral lung carcinomas, in addition to cytological sputum analysis, brush smears, transthoracic fine needle aspiration and transbronchial biopsies. BAL is a valuable diagnostic tool in cases of unusual pneumonia where fungi can be visualized by silver impregnation techniques and viruses by antibodies using immunofluorescence microscopy.