Standard dialysis fluid is not as pure as believed regarding chemical and microbial contamination. Although some current treatment devices are effective chemical purifiers, many are simultaneously good breeding grounds for bacteria. There is even some evidence that chronic exposure to low levels of both chemical and microbial contaminants can lead to the development of chronic diseases such as encephalopathy (aluminium) or amyloidosis (bacterial derivatives). Regarding chemical purity, national authorities fail to set limits on some important augmenting pollutants such as pesticides, fertilizers and aromatic hydrocarbons. In addition to specifying contaminant-associated clinical consequences, the different presently used water treatment devices and systems are critically re¬viewed. Dialysis water should nowadays always be highly purified, i.e. levels of chemicals should comply with national standards, and the microbial quality should be such that the total bacteria count is < 100 CFU/ml and the endotoxin level is < 0.25 IU/ml. The persistent microbial threat to even final dialysis fluid makes the placing of an ultrafilter before the hemodialyzer highly recommendable.