Eosinophil cationic protein was elevated during the first week of life in tracheal aspirates from 11 preterm infants in whom bronchopulmonary dysplasia subsequently developed compared with 8 preterm and 8 term infants without bronchopulmonary dysplasia. Eosinophil cationic protein levels increased progressively with continued intubation in the infants with bronchopulmonary dysplasia but remained low in a comparison group of term infants. We suggest that eosinophils participate in the inflammatory process in bronchopulmonary dysplasia and may contribute to lung injury.