The results of chlorine dioxide use after 23 years: Beginning in 2001, the Johns Hopkins Hospital in Baltimore installed chlorine dioxide (CIO2) water treatment systems in its inpatient buildings. The hospital evaluated the safety and efficacy of CIO2 for Legionella control throughout the potable water system. The evaluation included monitoring impact on Legionella, CIO2 residuals, disinfection by-products, biofilm formation, corrosion, dialysis filtration systems, laboratory filtration systems and building water systems. The application of the CIO2 system is designed and installed to optimize the efficacy of the disinfectant being used in the water treatment process. After 23 years of operation, an extensive Legionella analysis was performed. The data collected clearly indicates the successful control and elimination of Legionella in Hopkins’ water supply system.