Cytopathologic examinations of tracheobronchial aspirates from 108 infants sampled during mechanical ventilation demonstrated a well-defined progression of cytologic changes in bronchial cells that could be divided into three classes. Seventy percent of infants with respiratory distress syndrome who developed bronchopulmonary dysplasia had pulmonary effluent cytology designated Class III; no infants with RDS but without BPD had these cytologic findings. Additionally, a temporal progression of events involving polymorphonuclear leukocyte and macrophage populations occurred in the absence of infection; these events were associated with duration of assisted ventilation and oxygen exposure. The technique described provides a useful way to monitor the progression of lung injury and repair and offers a cytologic method to predict and diagnose the development of bronchopulmonary dysplasia.