 Bronchiolitis obliteran syndrome, BOS, can develop after lung or hematopoietic stem cell transplantation, HSTT. It has similarities in histopathological features and clinical manifestations, but differs in its contributing factors and clinical trajectories. After lung transplantation, it is limited to the lung allograft, while after HSTT it is caused by systemic graft versus host disease, GVHD. Diagnosing BOS after HSTT is more difficult because it can also be caused by extra pulmonary GVHD, leading to sclerosis or inflammation in the fascia or muscles of the respiratory girdle. Treatment is usually empirical and no established effective therapies exist. This review discusses the similarities and differences between BOS after lung transplantation and HSTT, and outlines treatment strategies for each condition. This article was authored by Alan O'Glamville, Christian Benden, and Bergeron, and others.