Eight hundred fifty 11- to 14-year-old residents of nonfluoridated communities in Massachusetts and Connecticut, who were born between 1972 and 1975, were investigated in a case-control study of the possible association between enamel fluorosis and exposure to fluoride supplements, infant formula, and/or fluoride dentifrice. The effect of median household income, an indicator of socioeconomic status, was also examined. Clinical examination, using the Fluorosis Risk Index, a fluorosis index developed for this project, allowed cases and controls to be identified based upon the specific time period of exposure to the various sources of ingested fluoride. Risk factor exposure was assessed via a mailed questionnaire with a response rate of 80%. Mild-to-moderate enamel fluorosis was strongly associated with fluoride supplementation during the first six years of life (odds ratio = 4.0) and with median household income (odds ratio = 6.6). Subjects in the middle median household income group who had used fluoride supplements through the first six years of life had a 28-fold increase in the risk of fluorosis compared with unexposed subjects in the lower median household income group. An odds ratio of 1.7 associated with infant formula use was suggestive of an increased risk of enamel fluorosis as was an odds ratio of 2.9 associated with fluoride dentifrice use.