Five cases of nosocomial Legionnaires' disease which occurred over a five-month period were retrospectively investigated. Chart review showed that during the two- to 10-day incubation period before the onset of illness, all of the patients inhaled aerosolized tap water from jet nebulizers (four patients) or from a portable room humidifier (one patient), and all received high dosages of corticosteroids or adrenocorticotropic hormone. Exposure to both factors was highly significant (P less than 0.000001) when compared with the rate of exposure in 69 control patients. Environmental cultures yielded Legionella pneumophila from tap water and from reservoirs of tap water-filled respiratory devices. The yield was highest from hot tap water, in which the free chlorine level was less than 0.05 parts per million. Thus, Legionnaires' disease may be caused by contaminated aerosols from respiratory devices, and the use of contaminated tap water in such devices represents a previously unrecognized hazard to which corticosteroid-treated patients should not be exposed.