Sixty-five cases of nosocomially acquired Legionnaires' disease are reported and the world literature is reviewed. The etiologic agent, Legionnella pneumophila, has been isolated from several environmental sources at outbreak sites. Legionnaires' disease appears to be acquired by inhalation and is primarily manifested by severe, potentially fatal, pneumonia. Characteristic clinical disease consists of high fever with relative bradycardia, dry cough, chills, diarrhea, and pleuritic pain. Although no single feature is pathognomonic, the clinical presentation is usually sufficiently characteristic to suggest the diagnosis. The diagnosis of Legionnaires' disease during acute illness may be established by culture of Legionella pneumophila, or by demonstration of the bacterium using special stains. However, in most instances, the physician must make a presumptive diagnosis based on the clinical presentation in order to institute appropriate antimicrobial therapy. Retrospective confirmation of the diagnosis may be made by serologic studies in most instances. Erythromycin is, at this time, the drug of choice for the treatment of Legionnaires' disease. A prompt salutory response following institution of erythromycin therapy is typical.