This study regards the quality of the water used in 4 types of dental units making use of softened and heated water, softened but non-heated water, non-softened but heated water and non-softened and non-heated water. The samples were taken from the incoming tap water, from oral rinsing cup, the air-water syringe and the ultrasound descaling hand-piece. The results showed how the water underwent a notable growth in bacteria during its passage within the circuits of the units, reaching heterotrophic total counts greatly exceeding the guidelines set down by Italian laws regarding drinking water. While the influence of softening was evident, the bacteria in the samples taken from descaling handpiece, where there is more stagnation, found excellent growing conditions also at high temperatures. In the softened and heated waters a notable growth of Pseudomonas aeruginosa was found and it is likely that this was encouraged by the combined effect of the softening and heating. As far as the origin of the contamination is concerned, the bacteria present in the water systems seem to have come from the incoming water.