Both legionnaires' disease and pneumonia caused by Pittsburgh pneumonia agent (PPA) are endemic in the Veterans Administration Medical Center, Pittsburgh. 85% of cases of legionnaires' disease and 100% of the cases of PPA were acquired in hospital. A selective dye-containing medium which allows visual discrimination of Legionella pneumophila and PPA has been used in a large-scale environmental survey of the hospital. Samples from 53 sites, including hot-water storage tanks, showerheads, mixing valves, and taps from 19 wards, were cultured. PPA was isolated from 2 sites and L. pneumophila from 33 sites. Water from a tap in the surgical intensive-care unit yielded 6 colony-forming units (CFU) of PPA/0.1 ml and 300 CFU of L. pneumophila/0.1 ml after centrifugation. Water from the outlet valve of a hot-water storage tank yielded 10 CFU of L. pneumophila/0.1 ml and when concentrated by centrifugation also yielded 2 CFU of PPA/0.1 ml. PPA and L. pneumophila share the same environmental niche, but isolation of PPA is more difficult. It seems that the reservoir for PPA in the VA Medical Center is the hot-water distribution system. PPA and L. pneumophila were simultaneously isolated from the lung tissue of a patient in this hospital who died of hospital-acquired pneumonia; this finding supports the hypothesis of a common reservoir and mode of transmission for the two organisms.