The cooling and process water of dental unit waterlines is often exposed to a high bacterial load. In a first part of the study, the efficacy of treatment in newer units (<10 years of operation) installed in-line filters of Aqua free has been investigated. Before filtration it was shown always a load of Legionella and once with pseudomonads despite regular disinfection . After filtration, the by RKI (2006) recommended standards for Legionella and Pseudomonas could always be respected. In a second study section earlier dental unit waterlines were examined (> 10 years of operation) after installing the inline filter. It was first shown a significantly higher bacterial contamination than the newer treatment units. The effectiveness of the filter with regard to the reduction for Legionella and Pseudomonas has also been demonstrated in these units despite irregular renovations and unfavorable initial conditions. Because the filters are not fit terminal, the risk of retrograde contamination is given. This fact confirms the need to always use a combination of both chemical and physical measures.