A 9-year-old boy with secondary chronic myelogenous leukemia after treatment of acute lymphoblastic leukemia underwent allogeneic bone marrow transplantation (BMT) from an HLA-identical sibling in December 1998. Grade II acute GVHD developed on day 24 and chronic GVHD developed 5 months after BMT. Cough and dyspnea appeared 9 months after BMT. Despite administration of tacrolimus and methylprednisolone (m-PSL) pulse therapy, the dyspnea gradually increased in severity. Bronchiolitis obliterans was diagnosed on the basis of lung biopsy in January 1999. Because renal dysfunction made it difficult to continue the use of tacrolimus, we attempted antithymocyte globulin (ATG) + m-PSL therapy. Major BCR/ABL mRNA was transiently positive on RT-PCR after the ATG + m-PSL therapy, but no severe complications were observed. A decreasing V50/V25 level and increasing peak flow value were observed in respiratory function tests after ATG + m-PSL therapy, and the patient is currently free of dyspnea. Our findings suggest that ATG + m-PSL therapy is beneficIal for patients with drug-resistant bronchiolitis obliterans after BMT.