BACKGROUND: Because of the ubiquity of Legionella species in aquatic environments, molecular epidemiological analysis of Legionella isolates is important in investigation for source of infection and subsequent control of nosocomial legionellosis. In association with an unusual cluster of nosocomial pneumonia with Legionella in a tertiary-care hospital, we performed an environmental surveillance with molecular epidemiological study of Legionella isolates. METHODS: We randomly collected 20 samples of environmental and portable water from the hospital where three cases of Legionella pneumonia occurred consecutively during the period of 5 months. We detected Legionella from the samples by using both culture and polymerase chain reaction(PCR), and analyzed Legionella isolates from patients and environmental samples together with 12 reference strains by ribotyping using HpaI and EcoRI. RESULTS: Legionella was isolated from 3 out of 20(15%) samples by culture, and detected in 9 of 20(45%) by PCR. Ribotyping analysis showed that 2 patients' and 2 environmental isolates from a faucet of the patient's room and an air handling unit shared the same pattern which was also identical to that of Legionella pneumophila serogroup 6, a reference strain. CONCLUSION: The study showed that the hospital environments were contaminated with at least 2 Legionella species including L. pneumophila serogroup 6, and indicated that an unusual cluster of Legionella pneumonia occurred in the hospital was possibly linked to the contamination of a faucet with L. pneumophila serogroup 6.