We measured eosinophilic cationic protein (ECP) concentrations in the circulation and bronchoalveolar lavage (BAL) fluids from patients with chronic eosinophilic pneumonia, patients with eosinophilic granuloma, and normal control subjects. Significantly increased ECP concentrations were found in the circulation of patients with chronic eosinophilic pneumonia and with eosinophilic granuloma compared with those found in control subjects. The ECP concentrations were well correlated to eosinophil counts in the circulation of patients with chronic eosinophilic pneumonia, while they were not in patients with eosinophilic granuloma. Chronic eosinophilic pneumonia patients had prominently increased ECP concentrations in BAL fluids compared with those found in control subjects, while eosinophilic granuloma patients did not. Those concentrations in chronic eosinophilic pneumonia patients were well correlated to eosinophil counts in the BAL fluid. Corticosteroid therapy remarkably decreased circulating ECP concentrations in three patients with chronic eosinophilic pneumonia, but it had no significant effects in two patients with eosinophilic granuloma. Measurement of ECP concentrations seems to be useful to evaluate the disease activity of chronic eosinophilic pneumonia.