We report a patient who developed bronchiolitis obliterans organizing pneumonia (BOOP) after syngeneic BMT for ALL. The patient complained of persistent low-grade fever and non-productive cough after engraftment. Chest CT scan showed patchy infiltration bilaterally in the lower lung fields. Antibiotics were ineffective. Cultures, serological studies and polymerase chain reaction detected no infectious pathogens. We finally made a diagnosis of BOOP by thoracoscopical lung biopsy. The lung lesion disappeared in a month with corticosteroid therapy. While BOOP following allogeneic BMT has been reported, this is the first report after syngeneic transplantation.