The benefit of semiannual application of the sodium fluoride varnish Duraphat and the silane fluoride varnish Fluor Protection was studied in 11--13-year-old children with high caries activity and life-long exposure to fluoridated drinking water (1--1.2 parts/10(6)). Annual clinical and radiographic examinations were made of 62 children in the Duraphat group and 70 children in the Fluor Protector group. Fluoride varnish was applied semiannually using the half-mouth technique. After 3 years, for the Duraphat group mean total DMFS increments on the control side were 6.2 and on the test side 4.3 (P less than 0.001); for the Fluor Protector group the DMFS increments were 4.9 and 4.4, respectively (NS). The caries reductions were 30% and 11%. Since there were no differences between initial mean DMFS scores of the groups, it is possible that the increment in the Fluor Protector control side compared to the Duraphat control side was due to fluoride ions from Fluor Protector crossing the midline and providing protection on the control side as well. Therefore, we conclude that Duraphat proved effective, but definite conclusion of the effect of Fluor Protector cannot be made. Children with the highest DMFS increment on the control side (Duraphat) gained most from the applications.