Microbial quality of water in a dental unit is of considerable importance since patients and dental staff are regularly exposed to water and aerosol generated by the unit. Water delivered to a dental unit by the so-called independent water system is the water coming from a reservoir which, at the same time, is an initial part of dental unit waterlines (DUWL). Thus, microbiological quality of this water is extremely important for the quality of water flowing from dental handpieces. The aim of the study was to assess microbiologically the water contained in dental unit reservoirs. Water samples were collected aseptically from the water reservoirs of 19 dental units. Results concerning microbial contamination: potable water quality indices, and detection and isolation of Legionella species bacteria, were presented. Over a half of the samples did not comply with the norms for potable water. In 63.1% of the cases, the number of colony forming units (cfu/ml) and of coliform organisms significantly exceeded acceptable values. Enterococcus was not detected in the samples of examined water. Similarly, no Legionella were found in the samples of dental unit reservoirs water. Reservoirs as water supplies and initial segment of DUWL should be subject to protocol to eliminate microbial contamination and routine monitoring to guarantee an appropriate quality of water used in dental treatment.