Until recently, the viruses most often identified in lower respiratory tract infections among immunocompromised adults with neoplastic disease have been the herpesviruses (cytomegalovirus, herpes simplex virus, varicella-zoster virus, human herpesvirus-6) and adenovirus. These viral pneumonias occur with greater frequency and/or severity among these adults than among immunocompetent adults. During the last decade, mounting evidence has suggested that the RNA viruses that commonly cause lower respiratory tract disease in the general population, namely influenza virus, respiratory syncytial virus, and parainfluenza virus, are also an important cause of serious community-acquired and nosocomial lower respiratory tract disease among immunocompromised adults. Because these infections are associated with considerable morbidity and mortality and are potentially preventable and treatable, it is important that they be recognized and treated promptly. Various therapies have been tried with some reported success, including amantadine (specifically for influenza A), ribavirin, and intravenous immunoglobulin. The optimal therapy or combination of therapies for these infections among immunocompromised adults remains to be defined in controlled trials.