An increase in endemic rate of nosocomial Legionella pneumophila pneumonia prompted an investigation that revealed 16.2% (12/74) of patient care hot-water sites surveyed were culture-positive for L. pneumophila. No positive cultures were recovered from cooling towers, air intakes, or construction areas. Heat flushing of hospital hot-water outlets to temperatures greater than 60 degrees C for 30 min achieved a 66% reduction in positive Legionella cultures. After 4 1/2 months, different serotypes recurred in previously eradicated areas and there were new positive cultures. Continuous supplemental chlorination of the hot-water system (2 parts per million [ppm]) significantly reduced the number of culture-positive samples from 37.4% (43/115) to 7.0% (8/115) after 6 weeks (P less than .005). Of 30 sites surveyed 6 months after hot-water chlorination, 67% (20) were still culture-negative. Of those positive, 70% had less than or equal to 150 L. pneumophila/ml and 90% were from bathtubs. Adverse effects of chlorination on users and plumbing have not been seen. There have been no definite cases of nosocomial L. pneumophila in areas served by supplemental chlorine during the first 17 months of the chlorination project. Technology allowing tighter regulation of chlorine and use of silicates to control corrosion have made continuous hot-water chlorination a safe and effective option in Legionella control.