The alveolitis of pulmonary sarcoidosis is characterized by an intense, mononuclear cell infiltrate that probably precedes granuloma formation. Pathogenic mechanisms underlying pulmonary sarcoidosis can be shown by study of the mononuclear cells composing the alveolitis. Bronchoalveolar lavage has shown that the sarcoid lung is characterized by increased numbers of "activated" T-lymphocytes within the alveolar structures. In contrast to normal control cells, the lung T-lymphocytes of patients with sarcoid release the mediator, monocyte chemotactic factor, that probably contributes to the pathogenesis of sarcoidosis by recruiting blood monocytes to the lung, thus providing cellular building blocks for granuloma formation. Conventional monitors of the activity of pulmonary sarcoidosis, such as blood studies, pulmonary function testing, and chest roentgenograms, show little assessed by bronchoalveolar lavage or by histopathologic studies. In contrast, quantification of lavage T-lymphocyte populations and 67Ga scintigraphy of the chest provide a sensitive and specific means of assessing the activity of the alveolitis in pulmonary sarcoidosis and may provide a rational basis for therapy.