Topical application of 3% sodium monofluorophosphate once every 2 weeks for 1 year reduced the caries increment markedly in a group of Danish schoolchildren. The number of reversals seemed to be particularly high. The amounts of monofluorophosphate received by this treatment is low, compared with the amounts appüed by normal use of toothpaste containing monofluorophosphate. Despite this, the reduction in caries increment was higher than in most studies where monofluorophosphate toothpastes were tested