Great improvements in water treatment technology and the spread of ultrafiltration for cold sterilization have been the basic support for the development and diffusion of on-line dialysis treatments. Some 20 years ago, nephrologists recognized that the official standards for dialysis fluids were insufficient with respect to these new treatment modalities, and ultrapure water (bacteria <0.1 CFU/ml; endotoxin <0.03 EU/ml) was proposed as a reference. Today, ultrapure water is included in most guidelines and recommended standards, but there remains a need for harmonization between standards. To achieve and ensure these levels of purity, technology must be supported by commitment of resources to an active quality assurance programme with adequate maintenance, monitoring, cleaning, sanitizing and problem analysis procedures.