To assess the value of bronchoalveolar lavage (BAL) for diagnosis, understanding, and treatment of amiodarone-associated pneumonitis, we examined the results of BAL total and differential cell counts and phenotyping of lymphocytes in 22 patients with this lung disorder and in 33 normal subjects. Overall, the total cell count was found to be almost the same as that seen in control subjects; the macrophage population was significantly reduced, and the lymphocyte, neutrophil, and eosinophil populations were increased in absolute number and percentage. When results were analyzed individually, BAL data appeared to be distributed according to two patterns. In the first pattern, there was no abnormal lymphocytosis. In the second pattern a lymphocyte alveolitis was found in percentage and in absolute number. This lymphocyte alveolitis was present either alone or associated with neutrophil alveolitis or with eosinophil alveolitis. In the first pattern, despite the normal level of the lymphocyte population, the percentage of CD4 T-lymphocytes and the CD4:CD8 T-lymphocyte ratio were significantly lowered. In the second pattern the CD8 T-lymphocyte count was increased in absolute number and percentage, with a low CD4:CD8 ratio. In six patients relavaged two to four months after amiodarone withdrawal, there was a significant fall in alveolar lymphocytosis, but the progressive increase in the neutrophil population over time seemed to be associated with the seriousness and progression of the disease. Finally, these findings closely resembled those obtained in patients with hypersensitivity pneumonitis due to inhalation of organic dust and suggest that an underlying immunologic cell-mediated mechanism may play a role in this iatrogenic pulmonary disease.