In an attempt to place in perspective the role of Legionella pneumophila in human disease, the literature concerning this organism was reviewed. In the five years since the etiology of legionnaires' disease was elucidated, an impressive body of information has been gathered from retrospective investigations of illnesses of previously unidentified etiology and from studies of new cases of nosocomial or community-acquired infection caused by L. pneumophila. Topics on which considerable knowledge has been gained include the characteristics of L. pneumophila itself; the etiology, pathogenesis, and epidemiology of infection; the mechanisms of immunity to infection; the role of aerosols and potable water in the transmission of disease; the spectrum of clinical manifestations; the efficacy of therapy; the features of sporadic cases; and the rapid diagnosis of infection by direct immunofluorescent techniques and bacteriologic methods. A limited amount of research has also been done on six other species of Legionella, all of which have been associated with pneumonia in humans.