Transbronchial biopsy specimens from ten patients with eosinophilic pneumonia were analysed to see whether the tissue contained activated eosinophils or secreted eosinophil cationic protein. The specimens showed many infiltrated tissue eosinophils in the intraalveolar spaces, and thickened alveolar septae and perivascular areas. Most of these eosinophils stained clearly with degranulation by immunohistochemical studies with monoclonal antibody EG2 (for the secreted form of eosinophil cationic protein), indicating that they were activated eosinophils. On the other hand, although pleural tissue obtained at bullectomy performed after pneumothorax showed eosinophilic infiltration, EG2-positive cells were few. Activated eosinophils may play an important role in the development of pulmonary lesions of eosinophilic pneumonia.