The benefit of semi-annual applications of sodium fluoride varnish (Duraphat) and silane fluoride varnish (Fluor Protector) was studied in 11-13 year-old children with life-long exposure to fluoridated drinking water (1-1.2 parts/10(6)). Annual clinical and radiographic examinations were made on 67 children in the Duraphat group and 71 children in the Fluor Protector group. Fluoride varnish was applied semi-annually using the half-mouth technique. At the end of 2 years, the mean overall DMFS-increments on the control side and test side of the Duraphat group were 5.0 and 3.8 (p less than 0.01), respectively, and of the Fluor Protector group 3.7 and 3.3 (NS). The caries reductions were 24% and 12%, respectively. Since there were no differences between initial mean DMFS scores of the groups, it was assumed that lower increments in the Fluor Protector group were due to Fluoride ions crossing the midline and providing protection on the control side as well. When increments in the Duraphat control side and the Fluor Protector test side were compared, the caries reduction of Fluor Protector was 35% (p less than 0.01). Fluoride varnishes provide additional benefit even when fluoride intake from drinking water is optimal.