Ventilator-related lung pathology in 83 very low birth weight infants was analyzed from autopsy material obtained over a 5-year period. A consistent evolving pattern of change appeared in all infants who died after 10 days or more of ventilation. This included the persistence of simple, evenly distributed terminal air spaces lined by undifferentiated cuboidal epithelium and separated by evenly widened septa with hypercellular fibrous stroma and increased amounts of subepithelial elastic tissue. Radial alveolar counts confirmed arrested development of terminal respiratory units. Changes in small airways were conspicuously absent, in contrast to the classical descriptions of bronchopulmonary dysplasia in larger infants. A high proportion of babies with evolving disease had severe acute pulmonary interstitial emphysema of relatively late onset with a lethal course suggesting increased susceptibility of immature mesenchyme to barotrauma. The distinct histologic pattern and clinical course leads to the conclusion that this is a parenchymal variant of bronchopulmonary dysplasia with the pathology restricted to the terminal respiratory unit.