The number of eosinophils and the concentrations of eosinophil cationic protein (ECP), a specific granule constituent of eosinophil granulocytes, were measured in bronchoalveolar lavage (BAL) fluid from patients (n = 22) with idiopathic pulmonary fibrosis (IPF). The median recovery of eosinophils during lavage performance was 4% (range, zero to 49) of the nonepithelial cells and significantly increased compared with the recovery in healthy control subjects (less than 1%) and in control patients with sarcoidosis (1%; range, zero to 7). The median BAL fluid concentration of ECP was in IPF 13.3 micrograms/L (range, 2 to 118) and significantly increased compared with the concentrations in healthy control subjects (2.7 micrograms/L; range, less than 2 to 8) and in control patients (6.6 micrograms/L; range, less than 2 to 64). The BAL fluid concentrations of myeloperoxidase (MPO) were also significantly increased in IPF, indicating a local neutrophil activation. A significant correlation was found between BAL fluid ECP and MPO, suggesting a common activator of eosinophils and neutrophils. BAL fluid eosinophils and ECP correlated with the reduced diffusion capacity of the lung but not with vital capacity or forced expiratory volume. It is concluded that eosinophil activation is part of the inflammatory process in IPF. ECP and other cytotoxic eosinophil products may play a pathophysiologic role for the lung damage in this disease.