The aim of this study was to determine whether the extent of an initial artificial caries-like lesion was related to its rate of progress after two applications of APF (1.23% F) and exposure to artificial saliva during lesion formation in vitro. Treatment of small lesions (mean depth 65.7 μm) resulted in their reversal or arrest while the same treatment of larger ones (mean depth 127.4 μm) did not prevent their progress despite evidence of some remineralisation. This suggests that the caries inhibitory effect of fluoride is optimal in the small, early lesio