Approximately 600 pediatric patients prescribed palivizumab (Synagis) prophylaxis in usual practice according to the approved Korean product label will be registered into this observational study. Baseline data will be obtained at enrollment including demographics, gestational age, birth weight and underlying diseases and complications especially in regard to respiratory disease and cardiovascular disease. At routine visits for Synagis administration, which will occur according to usual medical practice, information on Synagis prophylaxis, concomitant medication, and adverse events will be collected for up to 30 days after the last administration of Synagis.