Members of the family Legionellaceae have emerged as important nosocomial pathogens. Bronchopneumonia is the typical illness and mimics other nosocomial pneumonias. On rare occasion, surgical wound infections may be due to legionellae. Legionella pneumophila is the species causing most human illness, but other species may predominate in individual institutions. Pneumonia is acquired by inhalation of environmental aerosols from water sources contaminated by Legionella sp. In the hospital setting contaminated water sources include potable water, heat-exchange systems, and cooling towers. Water temperature control and maintenance programs may prevent colonization of water systems. Should clinical cases and water colonization occur, aggressive suppression/eradication programs--by chlorination or heat/flush--must be instituted. Surveillance of pneumonias in high-risk patient groups is essential for early detection of a nosocomial outbreak. There has been no evidence of person-to-person transmission. Familiarity with diagnostic techniques and their pitfalls will permit institution of effective antimicrobial therapy.