Following the introduction of the first fluorosis index by Dean, a series of fluorosis indexes were introduced. While they may differ in the specific way fluorosis is categorized, they all nevertheless use the same underlying diagnostic signs--originally described by Dean, Black, and McKay--that were causally linked to the development of enamel in areas with above-optimum fluoride in the drinking water. Underlying the various fluorosis indexes is the belief that specific clinical diagnostic criteria, based upon established clinical signs, can be utilized to differentiate fluorotic from nonfluorotic enamel opacities. These criteria repeatedly have been substantiated in studies in which the presence of enamel fluorosis, identified by clinical differential diagnosis, has been associated with fluoride exposure history. Further, to whatever extent nonfluorotic opacities have been misdiagnosed as fluorosis, observed estimates of association derived from analytical studies will have been underestimated.