The humanised monoclonal antibody palivizumab has been developed for prevention of serious lower respiratory tract disease caused by respiratory syncytial virus (RSV) in infants at high risk; RSV is the most common cause of lower respiratory tract infections in infants. Palivizumab specifically inhibits an epitope at the A antigenic site of the F protein of RSV subtypes A and B. RSV replication was inhibited in nasal and tracheal aspirates from infants receiving palivizumab 15 mg/kg. Mean 30-day trough serum concentrations of palivizumab were consistently about 70 mg/L in infants receiving repeated intramuscular or intravenous palivizumab 15 mg/kg. This is above the target serum concentration of 40 mg/L estimated to reduce pulmonary RSV replication by >99% in animal studies. In a large multicentre trial in 1502 infants at high risk of RSV infection, intramuscular palivizumab 15 mg/kg more than halved the incidence of RSV-attributable hospitalisation to 4.8% compared with 10.6% in placebo recipients. In the same group of high-risk infants, palivizumab significantly decreased total days in hospital attributable to RSV infection, days with increased supplemental oxygen requirement, days with moderate to severe lower respiratory tract infections and the incidence of admissions to intensive care. It had no effect on the incidence or total number of days of ventilation. Palivizumab was well tolerated during clinical trials in infants at risk of RSV infection. The incidence of adverse events was similar in placebo (10%) and palivizumab (11%) groups. Fever, irritability and injection site reaction were the most commonly reported adverse events.