Résumé : Background: Palivizumab reduces respiratory syncytial virus (RSV) hospitalisations in high-risk infants. Those with severe bronchopulmonary dysplasia may require two seasons of prophylaxis. There is concern that this humanised antibody might cause an adverse immune response in a second season of use. Objective: To evaluate and compare the occurrence of anti-palivizumab antibodies and clinical adverse events in subjects receiving monthly palivizumab injections for a first and second season, and to assess frequency and severity of RSV disease in the two groups. Design and Patients: Subjects aged ≤2 years at severe risk for RSV disease were designated as first season (no previous palivizumab exposure) or second season subjects (received palivizumab in previous RSV season). Palivizumab injections (15 mg/kg) were administered monthly for up to 5 months. Anti-palivizumab antibody titres and serum palivizumab concentrations were measured; adverse events were recorded. Results: No first (n = 71) or second (n = 63) season subjects experienced a significant anti-palivizumab antibody response (titre ≥1:80). Serum palivizumab concentrations were similar for the two groups. Nine (12.7%) first season and 8 (12.7%) second season subjects experienced one or more serious adverse events; most were respiratory and all were considered to be not or probably not related to palivizumab. No deaths occurred during the study. Conclusions: Monthly palivizumab injections were not associated with adverse immune responses or adverse events in young children receiving palivizumab for one or two seasons. Children receiving palivizumab for a second season did not experience more severe adverse events than those receiving it for the first time.