In 2001, human metapneumovirus (hMPV) was discovered in young children with respiratory tract infection of unknown origin. In the two years since its discovery the clinical characteristics of this new virus have been clarified. In children, especially those younger than one year of age, hMPV is responsible for 5 to 10% of respiratory tract infections requiring hospitalisation; its clinical course is somewhat milder, but otherwise indistinguishable from respiratory syncytial virus (RSV) infection. Human MPV can also be found in adults, in influenza-like illnesses, but also as a cause of pneumonia. Especially in the latter cases immunosuppressive conditions may be present.