Since July 1976 when an outbreak of severe, acute respiratory disease of unknown origin occurred among attendees at the 58th Annual Convention of the American Legion in Philadelphia, great progress has been made, including identification of the causative organism, Legionella pneumophila. Clinical features include not only respiratory involvement but often abdominal complaints, encephalopathy, renal disease, and rhabdomyolysis. Erythromycin appears to be an effective antimicrobial agent. Epidemiologic studies have defined epidemics as well as sporadic cases. Legionella pneumophila has appeared as an opportunistic pathogen. Organisms have been isolated from air-handling equipment and evaporative condensers in four instances. At present diagnosis usually is based on the demonstration of a fourfold rise in antibody titer between serum specimens obtained 3 to 6 weeks apart. Recent data suggest that organisms may be shown in tracheal secretions using a direct fluorescent antibody procedure and that antigen may be shown in urine using the microenzyme-linked immunosorbent assay (ELISA) technique.