Bronchiolitis obliterans (BrOb) is a well-recognized complication of allogeneic hematopoietic stem cell transplantation (HSCT). It is associated with substantial morbidity and mortality in adult patients. However, the incidence and morbidity of this complication have not been well described in the pediatric population. We report our experience of BrOb in 216 pediatric allogeneic HSCT patients between 1 January 2001 and 31 December 2005. In total 18 of 216 patients developed BrOb during this time. The diagnosis of BrOb was based on pulmonary function abnormalities, radiographic findings or lung biopsy. In total 14 of 18 patients with BrOb received stem cells from unrelated donors. In total 17 of 18 patients received bone marrow as a stem cell source, and 1 received peripheral blood stem cells. All pediatric patients in this report had a known risk factor for BrOb, most commonly chronic GVHD (l8 of 18 patients). Additionally, 7 of 18 patients had either toxic lung injury or virally mediated pulmonary disease before the diagnosis of BrOb. With a median of 45.1 months of follow-up, the outcomes were 5 of 18 patients died of lung disease, 2 died of other causes, 3 had progressive lung disease, 6 achieved partial resolution of disease and 2 had stable disease. BrOb, while uncommon, is associated with considerable morbidity and mortality in pediatric HSCT.