Environmental monitoring and clinical surveillance for Legionella species were done for 12 months as recommended by the Allegheny County Health Department (Pittsburgh). The water system of a hospital was found to be colonized with Legionella pneumophila serogroup 5. Three patients with nosocomial L. pneumophila serogroup 5 disease were subsequently diagnosed after laboratory tests for legionellae were made available for all patients with nosocomial pneumonia. All serogroup 5 isolates from the hospital water matched the 3 patient isolates by pulsed-field gel electrophoresis (PFGE). Furthermore, isolates found in the water supply dating back 10 years showed the same PFGE pattern. In contrast, 12 L. pneumophila serogroup 5 isolates from eight other institutions had different PFGE patterns. Routine environmental cultures were important in stimulating the application of Legionella laboratory testing, which subsequently identified unsuspected patients with nosocomial legionnaires' disease.