AIM: The purpose of the present study was to evaluate the effect of an ultrafiltration system on the bacteriological water quality in dental units. DESIGN: A BIN-X UF-45R ultrafiltration system with a pore membrane of 0.03 microm was mounted at the water supply of six older dental units while six control units were disinfected with sodium hypochlorite according to a standard procedure. As the water quality in the test units deteriorated in spite of ultrafiltration the test units were subjected to chlorination at different concentrations several times during the test period. Outcome measure: The number of colony forming units (cfu)/ml in water was determined according to European Standards on water quality. Results: While the median number of cfu/ml in control units never exceeded 7.6x10(1) the median cfu/ml in test units increased up to >1x10(5) in 3-4 days every time chlorination of the units was interrupted. Conclusion: The ultrafiltration system mounted at the water supply for six dental units was not able to control the bacteria originating from the existing biofilm in the water lines and deliver water of an acceptable quality.