Time-resolved in situ atomic force microscopy has been employed to examine erosion progression in untreated and fluoride-treated enamel specimens during exposure to citric acid. Contact with the acidic reaction solution initiated the emergence and growth of dissolution pits in both the native and the fluoride-treated enamel. In native enamel, pits are first observed after 90 min exposure to the reaction solution, compared to 250 min in the case of the fluoride-treated enamel. These findings indicate that, within the constraints of this study, a single application of fluoride solution (1000 mg/L, 2 min) confers protection to the enamel surface against acid-mediated erosion. This paper also highlights the potential role intrinsic defects may have on the susceptibility of enamel to erosion and, in part, may explain why some people are more susceptible to acid erosion. (C) 2010 Elsevier Inc. All rights reserved.