OBJECTIVE Two cases of nosocomial legionellosis due to Legionella pneumophila serogroup 6 (Lp6) were identified in the intensive care unit. Both patients had a history of aspiration of nasogastric tube feedings, developed pulmonary infiltrates, had positive cultures for Lp6, had serological titer rises to Legionella, were treated, and recovered. METHODS Isolates of Lp6 from the potable water system and patients were characterized by DNA restriction enzyme analyses using pulsed-field gel electrophoresis (PFGE). RESULTS Water samples grew > 10(4) CFU/L of Lp6, and the same PFGE pattern was observed with the patient and water isolates. Potable water was used only for delivering medications and diluting feeding solutions given through the nasogastric tubes of the patients. Heat shock of the hot water system (140 degrees to 160 degrees F or 60 degrees to 70 degrees C, 4 hours) was performed and the temperature was maintained between 131 degrees to 140 degrees F (55 degrees to 60 degrees C). Surveillance over 18 months revealed a reduction in Legionella to < 10(2) CFU/L. CONCLUSIONS We speculate that nosocomial Legionella pneumonia occurred due to aspiration of nasogastric tube solutions diluted with tap water. A nursing practice change to use only sterile water to dilute feedings and flush medications for nasogastric administration was instituted. The hot water temperature at the faucet was increased to > or = 131 degrees F (> or = 60 degrees C) to control Legionella. No further nosocomial cases have occurred.