By the use of a new classification system of dental fluorosis, primary and permanent teeth were examined in samples of children born in areas with 3.5, 6.0 and 21.0 parts/10(6) F- in the water supplies. The severity of dental fluorosis was lower than in the corresponding permanent teeth, but a significant increase in severity was noted with increasing concentration of fluoride in the drinking water. The distribution of dental fluorosis within the dentition followed the same pattern irrespective of fluoride content of the water. Thus, a progressive increase in severity was noted from the anterior to the posterior teeth. This pattern is presumably determined by variations in enamel thickness. The much thinner enamel layer of primary teeth may also explain the lower degree of dental fluorosis in these teeth rather than a maternal barrier to fluoride. The degree of dental fluorosis observed in medium and high fluoride areas did not support the hypothesis that the rapidly growing skeleton of infants prevents fluoride-derived enamel changes.