This study surveyed Legionella in 805 samples taken from 18 hot water systems under operating conditions. The results were analyzed and discussed in relation to water temperature, legislative requirements and optimization of the systems. The temperature of most samples (71%) ranged from 45-60 degrees C. The highest levels of colonization by L. pneumophila were found at water temperatures from 30-35 degrees C. At temperatures above 50 degrees C there was a large decrease in the number of positive samples, as well as the number of Legionella in individual samples. However, L. pneumophila was found in some samples having a temperature of 55-60 degrees C. These results indicate that the legislative requisite temperature of 50 degrees C for hot water systems is insufficient. A system operating temperature of 55 degrees C might be a better optimum, given the economical and safety limitations of temperatures as high as 60 degrees C. If it is impossible or ineffective to use classical method superheating (70-80 degrees C) then it is necessary take into account the chemical decontamination of water.