Sequential tracheal aspirates from 39 neonates with hyaline membrane disease were examined to correlate cytological findings with the development of bronchopulmonary dysplasia (BPD). A total of 224 tracheal aspirates were examined from these infants, 15 of whom developed BPD as diagnosed by conventional clinical and radiological criteria. Hyperplastic and metaplastic epithelial changes were observed in all infants studied. Dysplastic changes occurred in 14 of 15 who developed BPD, and 14 of 24 who had a normal outcome (P less than 0.02). The exfoliation of dysplastic metaplastic bronchial cells was thus 95% specific and 71% sensitive for the subsequent development of BPD. Factors associated with the development of BPD were very low birth weight and gestational age, persistence of a patent ductus arteriosus, high peak inspiratory pressure (cm of H2O/kg), prolonged assisted ventilation, and rapidity of development of class III (dysplastic) changes in tracheal aspirates.