OBJECTIVES: Hemodialysis (HD) is the standard of care for end stage renal disease patients. Reverse osmosis (RO) technology was designed to improve HD water safety. Strict guidelines regulate RO machine infection control. If required standards are not met, the machine is pulled Out Of Service (OOS), with repeated disinfection and culture performed before it can Return To Use (RTU). With repeated disinfection failures, purchase of a new machine may need to be considered; the cost-effectiveness of decision options is unclear. METHODS: A Markov model estimated the cost effectiveness of continued disinfection for a RO machine repeatedly failing to meet standards vs. purchase of a new machine over a 5 year machine lifespan. The model considered probabilities of remaining OOS after disinfection and of patient infection after RTU post disinfection, as well as costs of disinfection and RO purchase. Costs per patient infection avoided were calculated, taking a hospital perspective and testing result robustness in sensitivity analyses. We estimate that hospitals would pay at least $1 million per dialysis-related infection avoided, due to infection costs and the potential costs of liability and diminished reputation. RESULTS: In the base case, new machine purchase after 5 failed disinfections was least costly and least effective. Compared to this strategy, purchase after 4 failed disinfections cost $682,000/infection avoided. Purchasing after 3, 2, or 1 failed disinfection cost $2.1 million, $6.1 million, or $15.9 million per patient infection avoided, respectively. Purchase after 5 failed disinfections was favored if patient infection risk was substantially lower or new machine purchase cost >$14,365 (base case $10,000).   CONCLUSIONS: RO machine purchase after 4 failed disinfections is a favorable strategy at a $1 million/patient infection avoided threshold. Purchase after fewer failed disinfections may be favored if willingness-to-pay thresholds are higher.