Three patients with acute leukemia who underwent autologous bone marrow transplantation (BMT) in complete remission, developed a severe respiratory syncytial virus (RSV) pneumonia, which was fatal in two. Identification of RSV was made on the products of bronchoalveolar lavage by direct immunofluorescence. As already described by others, the initial course of RSV infection varies, depending on whether it occurs sooner or later after BMT with a better prognosis in the latter situation. Treatment consists of aerosolized ribavirin. Infection by RSV is caused by manual contact with infected persons and contaminated surfaces. The severity of lung RSV infection in the course of BMT suggests the need for prophylactic measures in addition to standard isolation precautions.