As a result of undocumented observations that the prevalence of dental fluorosis in both fluoridated and nonfluoridated communities may be higher than would be predicted on the basis of Dean’s data from the 1940s, dental fluorosis assessments using a modification of Dean’s Index were made in 1981 as part of routine examinations in a series of clinical trials. A total of 1,663 children in fluoridated or nonfluoridated communities, ranging in age from seven to 17 years, were examined during 1981-82. The prevalence of dental fluorosis in nonfluoridated communities ranged from 1.7 percent in 16-year-olds to 13.9 percent in 10-year-olds and, in fluoridated communities, ranged from 17.1 percent in 13-year-olds to 33.0 percent in 14-year-olds. At all age levels common to the two types of communities, the difference in prevalence of dental fluorosis was statistically significant. Compared with findings in Dean’s studies in 1942, for children of comparable age in communities with essentially the same water-fluoride levels, the prevalence of dental fluorosis in the present study was 3 1/2 times higher in nonfluoridated communities and two times higher in fluoridated communities. Mean fluorosis scores, however, were similar. If additional studies substantiate that the prevalence and intensity of dental fluorosis are increasing, the accepted norms for fluoride dosage need to be reassessed–especially in supplements, dentifrices, and water.