In 5 untreated patients with eosinophilic pneumonia of unknown origin, bronchoalveolar lavage (BAL) fluid was characterized by a markedly increased number of eosinophils (mean +/- 1 SD:42 +/- 22% of effector cells), which reverted to normal during treatment with corticosteroids. These findings indicate that besides the classic patterns of increased lymphocytosis or polymorphonuclear leukocytosis, a predominantly eosinophilic pattern also may be present in the BAL fluid.