The aim of the present study was to develop a simple disinfection method for reducing the content of bacteria in the water system of dental units to an acceptable level. The study was carried out at the Royal Dental College, Copenhagen, on 250 dental units. Samples of the cooling water supplying the ultrasonic scalers and of the water supplying the water glasses were obtained from eight different units representing different parts of the school. Disinfection of the water system was carried out by addition of chlorine to the pipe water near the institution's main water intake. The chlorination of the water was automatically regulated, and the installation was so flexible that the concentration of chlorine and the time and frequency of the chlorination could be varied. Different modes of chlorine dosage were examined. Before chlorination, the bacterial content in the water system of the units was about 10(4)-10(5) cfu/mL. It was found that an intermittent chlorination with 0.5-1 ppm chlorine for 10 minutes every day could reduce the normal bacterial counts in the water system to about a few hundred per mL.