This study examined the bacteriology of dental air-water syringes, and found that the water delivered by these syringes can be persistently contaminated with bacteria. Flushing of the water line reduced but did not eliminate this contamination. Even after six minutes' flushing, some water samples still contained more than 10(4) viable bacterial cells per milliliter, although coliform counts were less than two per 100 milliliters as measured by a Most Probable Number assay. Sterilization of the tip or the entire syringe did not eliminate this contamination. Scanning electron microscopy revealed bacterial biofilms on the inner wall of the plastic tubing supplying water to the air-water syringe, but not on the air line or on new, unused tubing. Such biofilms probably can be found in any tubing which supplies water to components of dental units. The inoculum for these biofilms comes from ubiquitous environmental aquatic bacteria, some of which can cause disease in compromised patients. Biofilms can be held in check by regular flushing of the water lines with a biocide, and the authors affirm the use of the ADA guidelines for treatment of contaminated dental-unit water lines to control possible cross-infection.