Until a few years ago, the pathogenesis of hypersensitivity pneumonitis was based on epidemiological, clinical and histological data. The introduction of bronchoalveolar lavage as a diagnostic tool in pneumology and the use of monoclonal antibodies have afforded considerable advances in pathogenic studies on this clinical entity. In the present work we review the characteristics of bronchoalveolar lavage in patients suffering from hypersensitivity pneumonitis in order to study the mechanisms through which the disease appears after integrating such characteristics with the classical histological, clinical and biochemical data. Bronchoalveolar lavage in these patients demonstrates the following: An elevation in the number of total cells that is higher than in any other interstitial pathology. Morphological, biochemical and functional data suggestive of activation of alveolar macrophages. An elevation in lymphocyte percentages with data characteristic of activation. The rise occurs in the two lymphocyte subpopulations (CD4 and CD8), although there is a predominance, in the rise in C8 lymphocytes, such that the CD4/CD8 ratio decreases. A rise is also observed in "natural killer" cells. Functionally, there are data that reveal an increase in the inducer potential of T lymphocytes together with an increase in cytotoxic capacity, in all their subpopulations. The results on the lymphocyte count vary broadly according to the course of the disease. Varying increases in neutrophils, eosinophils and mast cells. The rises in the first two kinds of cells indicate recent contact with the antigen, while the rise in mast cells seems to be of pathogenic significance. Different biochemical alterations (complement system, immunoglobulins, lipids, etc.).(ABSTRACT TRUNCATED AT 250 WORDS)