Between August 1978 and November 1983, 21 cases of pneumonia caused by Legionella pneumophila occurred in the Leiden University Hospital, mainly among immunocompromised patients. A new serogroup of L. pneumophila, designated serogroup 10 (prototype strain Leiden 1), was isolated from bronchial secretions of four patients, and five patients had serological evidence of infection with this organism. Nine patients had a culture-confirmed infection with L. pneumophila serogroup 1. L. pneumophila serogroups 1 and 10 were also isolated from the hot potable water supply in the building to which 19 of the 21 patients had been admitted. The isolates of L. pneumophila serogroup 1 from patients and the hot potable water were identical in studies with monoclonal antibodies and had the same plasmid profiles. These findings provide further evidence that in our hospital potable water contaminated with L. pneumophila is a source of infection, mainly in immunocompromised patients.