Bacterial contamination of the wate supply of newly dental units was investigated. Water samples were collecged from water supply lines to the dental operatories prior to connection of the dental units. Within hours following connection, and continuing for up to the 6 months of the study, water samples were obtained from the air-water syringe of the units. The samples were serially diluted 10-fold and plated on culture media for quantitative analysis. The formation of bacterial biofilm in the dental water supplying tubing was monitored by scanning electron microscopy. The results of these studies revealed that the building's water supply to the dental units was contaminated prior to connection to the units. The water supply from the air-water syringe was therefore contaminated as well. The number of contaminant bacteria in the dental unit water supply increased for several weeks and then stabilized. The lumen of the dental tubing became progressively contaminated with bacterial biofilm, which subsequently became the primary reservoir for maintenance of the contamination of the dental unit water suply.