To estimate the total contribution of water fluoridation to caries reduction by including the benefit from the diffusion of fluoride from fluoridated communities to surrounding nonfluoridated communities via the export of bottled beverages and processed foods. Methods: We analyzed data from the 1986–87 NIDR Children's Survey for 18 507 school children aged 6–17 years who had at least one permanent tooth and for whom a complete fluoride exposure history could be created. To measure water fluoridation exposure, we generated continuous and categorical exposure variables. Years of fluoridation exposure (YFE-continuous) measured the number of years the child lived at residences receiving fluoridated water. Lifetime fluoridation exposure (LFE-categorical) was high if the child lived at residences receiving fluoridated water more than 50% of his life and low, otherwise. We summed the proportion of state population receiving fluoridated water times the number of years the child had lived in each state and then divided this value by the child's age to measure diffusion exposure (DE). We grouped DE into three levels: low (DE