An intraoral cariogenicity test (ICT) was used to study the enhancement of fluoride treatments (2% NaF or 8% SnF2) that resulted from prior cariogenic challenge to human enamel surfaces. Measurements of enamel microhardness and microradiography indicated higher resistance to the cariogenically primed than for the unprimed surfaces. These results are interpreted as an adaptation of enamel to a challenging local environment by the formation of highly caries resistant minerals in the microspaces created by the priming challenge. With this adaptive reaction, tooth surfaces at risk were converted to surfaces of higher resistance than nonprimed surfaces. It is suggested that the concept of enamel adaptation to the cariogenic challenge can be applied clinically to arrest incipient caries, especially in patients who respond well to preventive disciplines that include the use of fluoride treatments.