abstract — The purpose of this study was to test the retention as well as the caries reduction potential of a fissure sealant under Finnish dietary conditions. The 150 children who served as subjects were selected on the basis of having at least one pair of sound permanent molars. Two hundred first molars, 43 premolars and 119 primary molars were sealed, while the respective tooth from the other side of the jaw was left untreated and used as the control. The findings after 6 months revealed that all 362 sealants persisted. Among the sealants of the first permanent molars, 99% were in excellent condition. All sealants of the permanent premolars and of the primary molars were excellent. Among the permanent control molars 43 (22%) showed carious fissures while among the sealed permanent molars, only 3 (1.5%) had carious fissures. This gives a 93% reduction of the fissure caries. Both the control and the sealed premolars were all sound. None of the sealed primary molars were decayed compared to 7.6% decayed on the control side.