The clinical retention of a single application of a pit and fissure sealant was evaluated on the teeth of children to determine the influence that this protective coating, or its loss, would have on caries activity during a four-year period. The sealant was fully retained on 50% of all paired permanent teeth at 48 months. Of the 689 pit regions that retained the sealant, 95% were rated as having severe loss of substance, only 5% as slight loss, and none with no loss of substance. It also was found that 53% of all paired permanent control teeth judged cariesfree at the baseline examination were found to be carious, whereas only 30% of the treated teeth examined were carious. The corresponding relative reduction in caries rates was, therefore, 43%. The actual number of teeth estimated to be saved by the sealant treatment at 48 months, as measured by net gain per 100 teeth treated, was 23. The net gain per child was estimated as 2.7 teeth saved from caries (full mouth), with a positive treatment benefit in 69% of the children. When the sealant remained intact, there was a pronounced reduction in caries initiation--the percent effectiveness being 84%.