Seventeen children with oxygen-dependent bronchopulmonary dysplasia, right ventricular hypertrophy, and Doppler echocardiographic evidence of pulmonary hypertension were studied by cardiac catheterization. Fifteen of these patients had pulmonary hypertension when placed in room air; six of these 15 patients were shown to have large systemic-to-pulmonary collateral vessels. The hemodynamic responses to oxygen and hydralazine were evaluated. Five patients developed normal pulmonary artery pressure while receiving supplemental oxygen and were not studied further. Of the remaining ten patients, the six patients with large, hemodynamically significant collateral vessels all had deleterious reactions to hydralazine. Two of the four patients without collateral pulmonary circulation responded to hydralazine with further reductions in mean pulmonary artery pressure. Five of the ten patients who had persistent pulmonary hypertension while receiving oxygen have died. Cardiac catheterization and angiography may provide important diagnostic, therapeutic, and prognostic information in patients with pulmonary hypertension complicating bronchopulmonary dysplasia.