In January 1977 an unsolved outbreak of infection at St. Elizabeth's Hospital (Washington, D.C.) that occurred in 1965 was linked with Legionnaires' disease. The link was made by fluorescent antibody testing with the bacterium isolated from tissues of persons with Legionnaires' disease in the 1976 outbreak in Philadelphia. In July and August 1965, an epidemic of severe respiratory disease characterized by abrupt onset of high fever, weakness, malaise, and nonproductive cough, frequently accompanied by radiographic evidence of pneumonia, affected at least 81 patients at St. Elizabeth's Hospital, a general psychiatric hospital. Fourteen (17%) of the affected patients died. Intensive epidemiologic and laboratory investigations in 1965 did not determine the etiology. The etiologic organism may have become airborne from sites of soil excavation.