Human metapneumovirus is a recently discovered pathogen that causes upper and lower respiratory tract disease in children. This study describes the course of illness in hospitalized children with this infection. During a 6-month period, 11 children were diagnosed with human metapneumovirus infection by reverse transcription-polymerase chain reaction. Oxygen supplementation was required for 82% of patients. Severe disease developed in 45%, and mechanical ventilation was required. An apparent life-threatening event was the indication for hospitalization of 27% of patients infected with human metapneumovirus. Children with underlying asthma or neuromuscular disease had a prolonged hospitalization.