MARKED inhibition of dental caries after 5 years of fluoridation has been demonstrated in a series of independent studies of school children (1-8). This report describes a similar finding in somewhat more minute detail. The children under study are residents of Prince Georges and Montgomery Counties, Md., immediately adjacent to the District of Columbia. All are white; all were born and have resided continuously in the area, and none has received such caries-preventive treatment as the topical application of fluoride solutions. Characteristics of the population, methods employed, and some of the techniques of analysis have been described in detail in previous publications (9, 10). In brief, teeth were examined under adequate light with mirror and explorer by a small and carefully calibrated group of examiners. Catches were not counted as carious lesions in the absence of other indications of caries. Examinations have been made yearly since 1952, when the community water was fluoridated. The fifth year results are presented in this paper. In 1952 the Washington Suburban Sanitary Commission* maintained two filtration plants, which supplied the water used by the children in the study. The average daily fluoride content of finished water at one plant was 0.98 ppm during the first year and 0.94 ppm during the second; fluoride content of finished water at the other plant was 0.90 ppm during both years. From January 1954 to January 1957, the average daily determination was 1 ppm fluoride at both plants, with most monthly averages within the range of 0.97 to 1.03 ppm fluoride. Since July 1955, all water has been processed in a single plant. The fluoridating vehicle has been sodium fluosilicate. Mean numbers of teeth in eruption were virtually the same in 1957 as in 1952. There were 1.2 percent fewer deciduous teeth present and 0.01 percent fewer permanent teeth present in 1957 than would have been expected on the basis of 1952 eruption data.