Seventy-three infants died after intensive respiratory therapy that included supplemental oxygen; their survival ranged from three to 225 days. Pulmonary alterations began with an exudative, early reparative stage and progressed to subacute and chronic fibroproliferative stages that correlated with stages 2 through 4 of bronchopulmonary dysplasia. Cardiopulmonary complications of interstitial fibrosis, emphysema, and cor pulmonale were most pronounced in the longest survivors and were associated with progression of the reparative response to the chronic fibroproliferative stage. An obliterative bronchiolitis was also a frequent manifestation of this reparative response, which may account for cystic bronchiolectasis observed in five infants. Ultrastructural studies indicated that the type 2 pneumocyte contributed to the reparative and proliferative reactions by reepithelialization of damaged septal walls and incorporation of hyaline membranes into septal walls that seemed to stimulate interstitial fibroplasia.