Treatment with ribavirin was instituted in 12 allogeneic and one autologous bone marrow transplant (BMT) recipients with proven respiratory syncytial virus (RSV), influenza B virus or parainfluenza virus infections. RSV was diagnosed in six cases, influenza B virus in four and parainfluenza virus in three patients. Ribavirin was given orally or intravenously (15-20 mg/kg/day in three divided doses) and in nine cases with the addition of ribavirin inhalations (6 g/day). Three patients required ventilator support. Three out of seven patients with pneumonia, including one patient with RSV who developed pulmonary infiltrates 10 days after the start of therapy, died despite treatment with ribavirin (two RSV, one influenza B). Multiple etiological agents were found in the fatal cases. The clinical condition improved in 10 of 13 patients during therapy. No serious adverse effects of systemic ribavirin were noticed. Two patients had reversible signs of hemolysis but only one patients required more erythrocyte transfusions than expected after BMT. Obstructive respiratory distress was often observed (6/9 patients receiving ribavirin inhalation therapy), which resulted in discontinuation of aerosolized therapy in four cases. Time to engraftment (WBC < 0.2 x 10(9)/l) did not differ from other non-treated BMT patients. We conclude that ribavirin is well tolerated both orally and intravenously and it may, if instituted before development of hypoxia, reduce morbidity and mortality of RSV, influenza B and parainfluenza in this group of patients.