Respiratory syncytial virus (RSV) infections in adult BMT recipients are frequently complicated by fatal pneumonias. Therapy of RSV pneumonia with aerosolized ribavirin alone has been reported to be associated with a 70% mortality rate. Because immune globulin therapy has been reported to be beneficial, we conducted a prospective trial of combination therapy with aerosolized ribavirin and intravenous immunoglobulin (IVIG). Aerosolized ribavirin was administered at 20 mg/ml for 18 h a day and IVIG was administered at 500 mg/kg every other day for the length of ribavirin therapy. Four lots of IVIG were chosen with RSV microneutralization Ab titers of 1:2048 to 1:8102. Between 8 January and 3 March 1993, during a community outbreak, 19 (45%) of 42 hospitalized adult BMT recipients with an acute respiratory illness were documented to have RSV disease. Two-thirds of these infections were hospital-acquired. All 19 patients presented with signs and symptoms of an upper respiratory tract illness. Sixteen patients developed pneumonia. The mortality was 22% in nine patients with pneumonia in whom therapy was initiated prior to the onset of profound respiratory failure. In contrast, the mortality was 100% in three patients with pneumonia in whom therapy was initiated within 24 h of respiratory failure requiring mechanical ventilation and in four untreated patients. We conclude that RSV may cause devastating outbreaks of severe pneumonia among hospitalized adult BMT recipients. Early diagnosis and combination therapy with ribavirin and IVIG was associated with a favorable outcome.