We report two cases of pulmonary venoocclusive disease (PVOD) in children with acute lymphoblastic leukemia treated by marrow allograft transplantation following conditioning with high-dose 1-3 bis chloroethyl-1 nitrosourea (BCNU), etoposide (VP-16), and cyclophosphamide (Cy). Both patients developed symptomatic pulmonary hypertension documented by right heart catheterization. Open-lung biopsy of one patient demonstrated PVOD evident even on frozen sections stained with hematoxylin and eosin. High-dose methylprednisolone was associated with significant clinical improvement in both patients. Pulmonary symptoms resolved in one patient who subsequently died in leukemic relapse. PVOD resolved in the other patient, only to recur when steroids were discontinued and then again respond to reinstitution of therapy. More aggressive therapy for malignant diseases may increase the incidence of PVOD. Prompt recognition of its subtle clinical and histological manifestations allows early institution of steroid therapy, which may be beneficial.