Intravenous palivizumab (15 mg/kg) was investigated in 2 phase 1 studies among recipients of hematopoietic stem cell transplants (HSCTs). Study 1 included 6 HSCT patients without active respiratory syncytial virus (RSV) infection. Study 2 included 15 HSCT patients with RSV upper respiratory tract infection (URTI; n=3) or RSV interstitial pneumonia (IP; n=12), all of whom also received aerosolized ribavirin. Peak serum concentrations of palivizumab in the 2 studies were similar. The mean serum half-life was 22.4 days in study 1, which mainly included autologous HSCT recipients, and 10.7 days in study 2, which mainly included allogeneic HSCT recipients. No antibodies to palivizumab were detected in study 1. No adverse events were attributed to palivizumab in the 2 studies. In study 2, all 3 patients with RSV URTI recovered without progression to lower respiratory tract disease, and 10 (83%) of the 12 patients with RSV IP survived the 28-day study period. Thus, palivizumab appears to be safe and well tolerated in HSCT recipients.