Although tissue damage, inflammatory cytokines, and an allogeneic ''graft vs. lung'' effect have all been implicated in the development of IPS and BO, the precise etiology of these syndromes is not well-defined. This study implicated common RV as a major risk factor for alloimmune lung syndromes. It is possible that these infections enhance local inflammation or changes that make the lung a target for allogeneic recognition. This is supported by the protective effects of acute graft vs. host disease perhaps related to the prolonged use of immunosuppression. This study is important for several reasons. The high incidence of respiratory virus infection (50%) is concerning in itself and may support the use of reverse isolation precautions so common in many transplant centers. This study suggests a possible mechanism of pulmonary toxicity after allogeneic transplant and should lead to further prospective studies for early monitoring, protection, and intervention of patients undergoing transplant. It would also be of interest to determine if early common RV infections occur with this frequency and are associated with pulmonary toxicity and adult patients undergoing HSCT.