Sixteen Japanese children, inadvertently exposed to drinking water containing 7.8 ppm fluoride (F) from birth, were aged 11 to 42 months when a low-F water supply was substituted. The appearance of the enamel of their permanent teeth was assessed 11 years later (children aged 12-15 years) and recorded using Dean's and the FDI indices. All grades of Dean's classification were seen. The enamel lesions were more severe in the older than in the younger children, in the incisal compared with the gingival halves of the teeth, and in the early- compared with the late-forming teeth. Small, localized, chalky-white areas of enamel were found at or close to the incisal margins of the anterior teeth of six children. The characteristics and the distribution of the lesions in this study add to our knowledge of the mechanism by which fluoride produces dental fluorosis.