Of 21 cases of legionellosis, 14 were of nosocomial origin and 6 others had a possible nosocomial source. The hot and cold water systems, respiratory therapy equipment, puddles of rain water on flat roofs and in gutters, and demineralized water systems were all excluded as a source of nosocomial infection. By subtyping it was shown that Legionella pneumophila serogroup 1 isolates from 11 patients and from the cooling towers were indistinguishable. Because of this result the cooling towers were considered to be the infection source. After effective chlorination of the cooling towers, the frequency of legionellosis declined.