Using morphometry, the pulmonary vasculature in lungs obtained at autopsy from 8 patients with bronchopulmonary dysplasia (BPD) was studied. In these specimens the axial arterial pathway was similar in length to those of equivalently aged fetuses and increased with lengthening survival. The internal diameter of axial arteries was variable--excessively wide in 2 patients and diffusely narrow in 3. Microscopically, the percent medial thickness of muscular pulmonary arteries was reduced compared to fetal values and tended to be less in older patients. Compared to normal fetuses, there were more muscularized intraacinar arteries, suggesting peripheral extension of smooth muscle. Arterial concentrations were elevated in 2 long-term survivors. The weight of the cardiac right ventricle was reduced in 4 patients. These findings indicate that in patients with BPD there is a complex dual process of pathological remodeling and an attempt at normal anatomic adaptation of the pulmonary vasculature to extrauterine life.