After five semi-annual applications of two fluoride varnishes, the amount of permanently bound fluoride in the enamel surface was determined in children with lifelong exposure to fluoridated water. The experimental groups consisted of 51 (Duraphat®) and 61 (Fluor Protector®) children aged 13–15 years; and a control group of 71 children of the same age with no history of topical fluoride treatments. 6 months after the 5th semiannual application, in vivo determinations of fluoride content were made on the buccal surface of the upper first premolar using the etching method of Hotz et al. Mean fluoride content at an average depth of 2.9 μm was 3,364 ppm in the Duraphat group, 4,179 ppm in the Fluor Protector group, and 2,036 ppm in the control group. The overall difference between groups was significant (p < 0.