Dental caries (i.e., tooth decay) is a transmissible, multifactor disease that affects 50% of children aged 5-9 years, 67% of adolescents aged 12-17 years, and 94% of adults aged > or = 18 years in the United States. During the second half of the 20th century, a major decline in the prevalence and severity of dental caries resulted from the identification of fluoride as an effective method of preventing caries. Fluoridation of the public water supply is the most equitable, cost-effective, and cost-saving method of delivering fluoride to the community. In the United States during 2000, approximately 162 million persons (65.8% of the population served by public water systems) received optimally fluoridated water compared with 144 million (62.1%) in 1992. This report presents state-specific data on the status of water fluoridation in the United States and describes a new surveillance system designed to routinely produce state and national data to monitor fluoridation in the public water supply. The results of this report indicate slow progress toward increasing access to optimally fluoridated water for persons using public water systems. Data from the new surveillance system can heighten public awareness of this effective caries prevention measure and can be used to identify areas where additional health promotion efforts are needed.