A 7-year-old child died with primary pulmonary hypertension only 6 months after the onset of symptoms. Autopsy demonstrated severe pulmonary arterial medial hypertrophy and intimal proliferation. This case emphasizes the rapidity with which pulmonary vascular obstruction may occur, death resulting from potentially reversible structural abnormalities. It also demonstrates the variability in pulmonary vascular pathology in primary pulmonary hypertension and the necessity of biopsying the lung in order to predict those in whom vasodilator therapy might be helpful.