Ten recorded epidemics of Legionnaires' disease are reviewed to gain a working perspective on the epidemiology of the disease. Salient features have included a summer-fall seasonality, a male predominance that may largely reflect increased exposure risk among men, and a striking absence of person-to-person spread. That the disease is spread primarily via the airborne route is well established; air-treatment and air-conditioning equipment has been implicated as the amplification and delivery system in four epidemics. Soils and excavation sites have been suggested as sources of the organism in at least one recorded epidemic. Evidence to date suggests that the Legionnaires' disease bacterium may be widespread in nature. More complete epidemiologic understanding must await development of improved microbiologic and immunologic tests.