Eosinophilic inflammation has been observed in the airways of patients with chronic obstructive pulmonary disease (COPD). A subset of patients clinically diagnosed as having COPD show a reversibility of airflow obstruction when treated with corticosteroids, and may consist of patients with features of asthma including reticular basement membrane thickening and eosinophilic inflammation. Twenty-five unselected patients clinically diagnosed as having COPD received a daily oral dose of 1.5 mg/kg body weight of prednisolone for 15 d to assess the relationships between the functional response to corticosteroids and the presence of features of asthma. Eosinophilic inflammation was characterized before the course of corticosteroid therapy by enumerating eosinophils in peripheral blood, bronchoalveolar lavage fluid (BALF), and bronchial biopsies, using EG2 monoclonal antibody, and by measurement of eosinophil cationic protein (ECP) in BALF. A response to treatment was defined by an increase in FEV1 of at least 12% from baseline values and an absolute value of 200 ml measured at the end of the treatment. Twelve of 25 patients responded to the treatment. By comparison with nonresponders, responders had a significantly larger number of eosinophils (p < 0.015), and higher levels of ECP (p = 0.013) in their BALF. The responders had a thicker reticular basement membrane than the nonresponders (p < 0.04). These results indicate that a response to prednisolone in patients diagnosed as having COPD might occur more readily in a subset of patients presenting with features of asthma.