Mixed alveolar interstitial pneumonia is a much-feared complication in bone marrow transplant recipients because it carries a high mortality. Many cases in which an etiological agent is identified are due to fungi or cytomegalovirus; rarely, other infectious agents such as Pneumocystis carinii or (para)influenzavirus are involved. In this report we describe a patient who received intense chemotherapy followed by autologous bone marrow transplantation for relapse of testis seminoma and who developed a severe alveolar interstitial pneumonia caused by respiratory syncytial virus in the early posttransplant period. The patient fully recovered after treatment with ribavirin.