Respiratory viral infections are the most significant cause of increased mortality and morbidity especially in immunocompromised people. These infections are increasingly recognized as being the cause of the failure of a graft or the cause of death in both solid organ and hematopoietic stem cell transplant recipients. Treatment with potent immunosuppressive medication is necessary for regulation in order to prevent rejection of solid organs and graft-versus-host disease. As a consequence of this therapy, infections are more common. Respiratory viral infections are the most frequent and serious complications after hematopoietic stem cell transplantation (HSCT). Despite increased methods of testing for viral pathogens, nearly 10% of pneumonia in HSCT recipients still remain "idiopathic pneumonia syndrome". Recently described human metapneumovirus could be one of the etiological agents of this syndrome.