An investigation of an outbreak of nosocomial legionella pneumonia revealed that the potable water of the hospital was contaminated with a distinctive strain of Legionella pneumophila (designated the Le-1 strain), which was by culture and serology linked with human illness. To examine the possible role of this water in this outbreak, eight healthy guinea pigs were exposed to an aerosol of concentrated potable water. Seven animals developed pneumonia due to the Le-1 strain of L. pneumophila. In one of these guinea pigs, the Le-1 strain was also isolated from the blood and spleen. These results provide support to the theory that exposure of humans to aerosols of potable water contaminated with L. pneumophila may cause legionella pneumonia.