Enamel mottling is strongly associated with the water fluoride level of the community water supply. About 32% of the variation in the mottled enamel scores of subjects aged 7 to 12 was attributable to their community's water fluoride level. Objectionable mottling (moderate mottling) occurred at 2.3 times the currently accepted optimum water fluoride level. Subjects with incomes less than $10,000 have slightly more mottling (mostly moderate mottling) than subjects from higher income families. It should be emphasized that the effect of gender, ethnic group, or family income on enamel mottling is small compared with the effect of water fluoride level. Although many other personal and water supply characteristics were evaluated, virtually none of the remaining variation in mottled enamel scores could be attributed to any of these characteristics.