Legionella pneumophila infection has been recognized as an important cause of pneumonia. However, epidemiologic baseline data about the occurrence of legionella-related pneumonia and the distribution of legionellae in aquatic environments are still not available in Taiwan. A survey conducted by the National Institute of Preventive Medicine found that 8.6% of 487 pneumonia patients studied were positive for the L. pneumophila serologic test (indirect fluorescent test, IFA) and 32.0% of the 25 water samples were positive for isolation of legionellae. In conclusion, this survey supported the existence of Legionnaires' disease in Taiwan, indicated a preliminary prevalence of L. pneumophila antibodies among pneumonia patients and identified the prevalence of legionellae in water environments.