This post marketing observational study (PMOS) was conducted in Japan during the 2013-2014 and 2014-2015 Respiratory Syncytial Virus (RSV) seasons to assess the safety and effectiveness of palivizumab for the prevention of serious lower respiratory tract infection caused by RSV in participants 24 months of age and under, who have an immunocompromised medical condition (e.g., combined immunodeficiency disease, antibody deficiency, or other types of immunodeficiency; HIV infection; recovering from organ or bone marrow transplantation; on chemotherapy; on high-dose corticosteroid therapy; on immunosuppressants) or who have Down syndrome.