Infection control is an important issue in the dental surgery but the potential hazards associated with contaminated dental water have received relatively little attention in recent years. The complex design of the equipment results in stagnation of water within the dental chair and subsequent amplification of contaminating environmental organisms, including pseudomonads and legionellae, to potentially hazardous levels. Immunocompromised patients may be at particular risk of infection. Very poor water quality with total bacterial counts above 10(4) ml-1 is unpleasant for all patients, and the dental chair supply should be of drinking water quality. In addition to these problems, bacteria and viruses may be aspirated from the oral cavity and contaminate the handpiece. Measures to reduce microbial contamination of dental chairs and equipment include flushing water through the chair's equipment at the beginning of each day; continuous or pulsed water chlorination, or application of biocides other than chlorine; provision of sterile bottled water in the system; and autoclaving handpieces between patients. Future dental chair design must attempt to resolve the problems associated with microbial contamination of the water supply and aerosols generated during dental procedures.