The eosinophil count and concentrations of eosinophil cationic protein (ECP) were measured in bronchoalveolar lavage fluid (BALF) from patients with idiopathic pulmonary fibrosis (IPF), pulmonary fibrosis associated with a collagen vascular disorder (PF-CVD), sarcoidosis, and healthy controls. The patients with IPF and PF-CVD showed significant increased eosinophil count and ECP levels in BALF compared with the controls. When the patients with IPF and PF-CVD were subclassified into chronic stable, progressive, and acute progressive subgroups in accordance with the observed progression of pulmonary dysfunction during the preceding 3- to 6-month period, those in the acute progressive subgroup showed significantly elevated recovered eosinophil count and ECP level, as well as recovered lymphocyte count and total protein, albumin, and type III procollagen aminoterminal peptide-related antigens (pIIIp) in BALF, compared with either of the other two subgroups. Multivariate stepwise logistic regression analysis revealed that, among these variables, only ECP and pIIIp significantly contributed to discrimination among the three subgroups differing in disease activity. These findings suggest that eosinophils are involved in the inflammatory process in pulmonary fibrosis and that the released ECP and other cytotoxic eosinophil products may contribute to the lung injury and development of fibrosis.