Five cases of Legionnaires' disease caused by Legionella dumoffii were identified within an 11-month period in a hospital in the Quebec City area. In four cases bacterial isolates were obtained from clinical specimens, and in one case seroconversion was demonstrated. All the patients had been admitted to hospital within 10 days before diagnosis. Two of the patients were immunosuppressed children. Only 1 of the 40 hot water samples from the hospital yielded L. dumoffii; however, 6 of 11 distilled water samples contained the bacterium. All the patients had been exposed to distilled water, four through respiratory therapy equipment and one through a room humidifier. Following the use of sterile distilled water in the apparatus, no further cases were identified. This is the first reported outbreak of Legionnaires' disease caused by L. dumoffii, and it is the first time that nosocomial legionellosis has been linked to contaminated distilled water in Canada.