Numerous facilities operating secondary water distribution systems serving sensitive populations are searching for the appropriate point of entry (POE) disinfection technology to reduce the occurrence of Legionella at distal outlets. Chlorine dioxide has been successfully used in Europe and is beginning to appear in the United States as a POE disinfection technology. Chlorine dioxide is an effective disinfectant capable of killing numerous waterborne pathogens including Legionella, E. coli, Cryptosporidium, Giardia, and others. The ability of chlorine dioxide to kill Legionella in drinking water is of great interest to hospitals, where susceptible patients can be at risk of contracting hospital acquired Legionnaires' disease. POE disinfection technologies have been deployed in an effort to control pathogens (particularly Legionella) within secondary distribution systems. Such technologies are relatively new and few data exists to demonstrate effectiveness in full-scale applications. A study was implemented at a 437-bed acute care hospital in Central Pennsylvania (Hospital). The Hospital operates a secondary distribution system that includes a 520,000 gallon covered reservoir and 10,000 feet of distribution piping serving 23 buildings over 14 acres. Municipally supplied water accounts for ~80% of the daily demand, while the remaining supply is from an on-site chlorinated well. Identification of cases of hospital-acquired Legionnaires' disease prompted the Hospital to implement an environmental monitoring program and disinfection to control Legionella in the water distribution system. Control of Legionella was first attempted by maintaining a free chlorine residual between 0.3-0.5 mg/L and using a thermal eradication method from April 1998 - May 2000, but was ineffective for long-term control, logistically difficult, and costly. Copper-silver ionization was considered, but rejected because of the cost to install the ionization units in the many buildings. Chlorine dioxide was selected because it could be installed centrally at the reservoir to disinfect the water in all buildings, required minimal maintenance, and had a positive review in the literature from European and laboratory studies. The chlorine dioxide system was brought on-line in June 2000. From June 2000 to January 2001, the Hospital continued environmental surveillance for Legionella in conjunction with daily chlorine dioxide residual measurement. In February 2001 a standardized sampling protocol was initialized that included monthly collection of 100 ml of water from at least two hot and cold distal outlets in eight buildings, the source water to each building, and the reservoir (~48 samples/month). Monthly monitoring included pH, temperature (C), chlorine dioxide residual (DPD method; mg/L), and Legionella culture (CFU/ml). Periodic analysis at select locations include total organic carbon (Hach test.n tube; ppm) and total disinfection byproduct formation (ion chromatograph; ppm). Includes 14 references.