A 32-month study was undertaken to determine the caries inhibitory effect of a weekly rinse for one minute with a solution of 0.2% NaF on the first permanent molars. Subjects who participated in the study were 295 school children aged from 5 to 13 years who were divided by random selection in two groups, test and control who rinsed with either 0.2% NaF solution or a placebo under double-blind conditions. Reductions in caries increments on first permanent molars (20.7% DMFT and 31.9% DMFS) were seen in the test group as compared with the control group. A preventive effect was also seen in all first permanent molars surfaces, being the major protective effect in approximal surfaces (52% reduction) followed by occlusal (36%) and bucolingual surfaces (16%).