New onset airflow obstruction after BMT is a relatively common complication and may be seen in as many as 11% of long-term survivors of allogeneic BMT with chronic GVHD. Bronchiolitis and, occasionally, obliterative bronchiolitis is seen in the majority of cases in which histopathology is available. The primary risk factors recognized are the presence of clinical chronic GVHD, administration of methotrexate as an immunosuppressive, and older recipient age. Improved control of chronic GVHD with effective agents such as cyclosporine likely will decrease the incidence of this airway disorder. The causes probably are multifactorial and donor cytotoxic T-lymphocyte interaction with host cells is a likely contributor in many cases. The clinical course is variable, but the process usually is fatal in cases with rapidly progressive or severe obstruction. Interventions are directed at immune suppression and at diagnosing and treating infections that frequently occur in association with the airflow obstruction.