As part of a case-control study of community-acquired Legionnaires' disease, several factors related to residential water distribution systems and public drinking water systems were studied in the homes of 124 patients with community-acquired Legionnaire's disease and in the homes of 354 controls. The presence of water reservoirs and hot water tanks was studied in residential systems. Factors such as deficient chlorine levels, pipe repairs and other work, water flow interruptions, the use of alternative water sources, inadequate cleaning operations in public water reservoirs, and the position of the home within the public network (and whether this location constituted an endpoint) were studied in public water supply systems. Levels of legionellae in domestic water samples were also measured. Although the use of water reservoirs and hot water tanks promotes colonization by legionellae in residential systems, none of the variables studied seems to increase the incidence of community-acquired Legionnaires' disease.