The prolonged prophylactic effect of three years’ daily supervised toothbrushing with a 0.22% sodium fluoride effect dentifrice was studied in 113 schoolchildren. During the trial the number of new carious surfaces in the test group (57 children) was smaller than in the control group (56 children); the difference was 20% in the first year, 34% in the second, and 48% in the third. The numerical reduction found was largest for the proximal lesions. During the first two years after the end of the trial the test group showed a 13% reduction of caries. This was due mainly to a difference in caries increment on buccal surfaces. This means that no persistent prophylactic effect of great practical value could be demonstrate