A 49-year-old Japanese woman was admitted to our hospital on 23 July 1998 because of leucocytosis noted at a medical check. Her leucocyte and platelet counts were 14·1 × 109/l and 637 × 109/l respectively. She was diagnosed as having chronic myelogenous leukaemia (CML) based on the presence of a Philadelphia chromosome (Ph), and a BCR–ABL fusion gene was demonstrated. She was initially treated with interferon-α but became depressed. After 6 months of interferon-α, allogeneic peripheral blood stem cell transplantation (PBSCT) was carried out. Her human leucocyte antigen (HLA)-matched sister was treated with 300 µg of granulocyte colony-stimulating factor (G-CSF) for 6 d, and 119·25 × 108 of CD34-positive cells (2·93 × 106/kg of recipient body weight) were obtained. After high-dose cyclophosphamide (60 mg/kg/d for 2 d) and total body irradiation (2 Gy twice daily for 3 d), PBSCT was performed on 18 March 1999. She received a short course of methotrexate and daily cyclosporin A to suppress acute graft-versus-host disease. Four days after PBSCT, 300 µg of G-CSF was administered, and her leucocyte and platelet counts improved. Her blood type changed to that of her sister and the BCR–ABL gene was no longer demonstrated by reverse transcriptase polymerase chain reaction. On 2 September, she complained of dyspnoea after walking. Her blood gas analysis showed severe hypoxia (PaO2 56·1 torr, SaO2 90%, PaCO2 32·0 torr). A computerized tomography scan of her thorax suggested decreased blood flow in both lower lobes of her lungs. Pulmonary blood perfusion scintigraphy confirmed this (left). Pulmonary digital subtraction angiography showed no thrombus-associated obstruction, but a reduction of blood flow in this area (right). A diagnosis of pulmonary veno-occlusive disease (PVOD) was made, and she was treated with pulse steroid therapy. Although her hypoxia improved transiently, she developed pneumonia and died of acute renal failure 1 month later. An autopsy showed severe pneumonia but the pulmonary vessels were normal. Although PVOD is very rare, it should be recognized as an important reversible complication of stem cell transplantation.