Ten dental units were surveyed for microbial contamination of the waterlines. All units were found to be colonized with bacteria and fungi at levels ranging from 9.0 X 10(4) to 4.1 X 10(5) CFU/mL. Undiluted povidone-iodine 10%, loaded in five experimental units for 12 hours prevented recovery of microorganisms for 3 to 14 days when used in combination with sterile water reservoirs. Use of sterile water reservoirs alone did not effectively reduce levels of microbial contamination in five control dental units.