To describe two cases of nosocomial legionellosis and discuss the epidemiology of this infection.

Potable water was collected from multiple sites. Patient and environmental isolates were characterized by the Legionella slide agglutination test and monoclonal antibody subtyping. Concordance among isolates was confirmed by pulsed-field gel electrophoresis (PFGE).

A 713-bed university-affiliated hospital.

There was widespread contamination of potable water with Legionella pneumophila during a period of major construction; cooling towers were without growth of Legionella. One patient's isolate was the same by PFGE as the environmental isolate collected from the water faucet in his room. Control measures included superheating water used in all patient care areas to 75°C for 72 hours and flushing superheated water through faucets and showers; cleaning shower heads with a sonicator washer; and raising the hot water storage tank temperature from 43°C to 52°C. After these interventions, repeat environmental cultures over the next 6 months were without growth of Legionella, and no further cases of nosocomial legionnaires' disease were documented. An association between legionnaires' disease and construction is postulated. Heightened surveillance and preventive measures may be warranted during periods of excavation on hospital grounds or when potable water supplies are otherwise shut down and later repressurized.