A 37-year-old man with acute myeloblastic leukemia in first remission developed ulcerative colitis and bronchiolitis obliterans organizing pneumonia (BOOP) 7 months after bone marrow transplantation (BMT) from an HLA-matched brother who suffered from severe Crohn's disease. BOOP occurred 20 days after idiopathic interstitial pneumonia, in the context of severe ulcerative colitis. Lung and colon biopsies showed no signs of CMV infection or GVHD. The patient was treated with oral methylprednisolone 1 mg/kg/day and his clinical status and chest X-ray improved slowly. Remarkably, the symptoms of colitis also resolved with prednisone therapy and he is now symptom-free. We hypothesize that ulcerative colitis may have been transmitted from donor to recipient (adoptive autoimmunity) and that it was complicated by BOOP. However, other factors such as CMV may have contributed to the occurrence of BOOP.