A subclinical inflammatory alveolitis as assessed by BAL cell analysis may be present in a high proportion of symptomless patients with immunological systemic disorders and with normal chest roentgenogram. Subclinical alveolitis can be characterized by the relative proportions of the different cell populations comprising the alveolitis and by the activated state of the cells. Thus, subclinical alveolitis can be classified into two major groups: lymphocyte and neutrophil alveolitis. Lymphocyte alveolitis is frequently found in patients with extrathoracic granulomatosis (Crohn's disease, primary biliary cirrhosis, extrathoracic sarcoidosis) or with some collagen vascular diseases (Sjögren's syndrome, rheumatoid arthritis, systemic lupus erythematosus). Neutrophil alveolitis is a main finding in collagen vascular diseases, especially progressive systemic sclerosis, dermatopolymyositis and mixed connective tissue disease. In addition, alveolar macrophages may be spontaneously activated and release various mediators that could be relevant to the pathogenesis of interstitial lung disease. On the other hand, some other alveolar macrophage functions (antibacterial activity may be severely impaired in some diseases, for example systemic lupus erythematosus). Alveolar inflammation is associated with an increase in the permeability of the alveolar membrane responsible for an increased influx of blood proteins in the alveolar spaces. Although subclinical inflammation may also be detected by high resolution computed tomography (HRCT) scan and/or lung permeability scintigraphic studies, the significance and prognostic value remains unclear and clearly differs according to both the disease and the pattern of alveolitis.