Abstract:Thirty‐four samples of warm waters from 12 psychiatric centres in Victoria and New South Wales were examined for legionellae by guinea‐pig inoculation.Legionella pneumophilawas isolated from 20 of the samples collected from ten of the establishments investigated. The detected prevalence proportion ofL. pneumophilain waters of temperatures 36 to 43oC was 0.9 (18/20), whereas the prevalence in waters of temperatures 45 to 54.2oC was 0.14 (2/14). The two ‘positive’ waters within the latter range showed evidence of low numbers ofL pneumophila.No significant antibody titres to relevant serogroups were detected in the 112 exposed residents tested from seven psychiatric hospitals in New South Wales.The findings show that the temperature range with the greatest prevalence ofL. pneumophilain warm waters is 36 to 43oC. The presence of legionellae in these warm water‐distribution systems contrasts with their absence from the water‐distribution systems of Victorian hospitals in an earlier survey and underlines the value and simplicity of the usual Australian practice of maintaining hospital hot water temperatures at about 70oC in the control ofL