Two cases of fatal veno-occlusive disease are described: the first in a boy 14 years old and the second in his sister, who died, at age 13, only a few months later. In both, the first symptom was effort-induced dyspnea and the course of the disease was rapid. The chest radiograph of the boy showed diffuse opacities, whereas that of the girl showed Kerley lines bilaterally. In both, at autopsy, a diagnosis of severe veno-occlusive disease was confirmed, with partial or complete occlusion of much of the pulmonary venous bed by organized thrombi. The pulmonary vascular system of the girl was processed for light microscopic morphometry by injection of radiopaque medium into the veins of one lung and the arteries of the other. Although there was little filling of the venous system, much of the arterial system filled down to the capillary level. Pre-acinar and intra-acinar arteries showed medial hypertrophy and extension of smooth muscle into precapillary arteries. The concentration of patent intra-acinar arteries relative to alveoli was not significantly less than normal, whereas the concentration of veins was strikingly reduced. Possible pathogenetic mechanisms are discussed.