The aim was to determine at what ages the different types of human permanent teeth are susceptible to development of fluorosis. The prevalence of disturbances of enamel mineralization was recorded blindly in 70 children who had previously participated in a fluoride tablet program (0.5 mg F–– per day) and 40 other children who had never received fluoride tablets. Comparison of the prevalence of enamel lesions between these two groups and between the ages at which children began to take the tablets revealed the ages at which there was an increased risk of fluorosis. The data showed that fluoride can affect teeth late in their development when enamel is in a stage of late secretion or early maturation. It was concluded that this finding, which is in accordance with experimental animal studies, calls for a fundamental change in thinking not only with respect to the mechanisms involved in human enamel fluorosis, but also in relation to timing of fluoride programs.