The quality of dental unit water is of great importance since patients and dental staff are regularly exposed to water from aerosols generated during work. The main purpose of this investigation was mycological evaluation of dental unit waterlines (DUWL). The author determined the number and species of fungi present in the water from a unit reservoir which is the source of water for a dental unit, in the water flowing from a high-speed handpiece of a unit, and in the swab sample collected from the wall of a waterline connecting a unit reservoir and dental handpieces. The following mould fungi were identified: Aspergillus amstelodami, Aspergillus fumigatus, Aspergillus spp. from Aspergillus glaucus group, Aspergillus repens, Citromyces spp., Geotrichum candidum, Penicillium aspergilliforme, Penicillium pusillum, Penicillium turolense, Sclerotium sclerotiorum; yeast-like fungi: Candida albicans, Candida curvata and other yeasts. Some of them, in certain circumstances, especially in people with immunological disorders, may be a cause of opportunistic infections. Thus, it is necessary that the DUWL should be submitted to a decontamination protocol and to routine microbial monitoring to guarantee an appropriate quality of water used in dental treatment.