Purpose: To evaluate sealant retention in a multi-operator school-based oral health programme and sealant efficacy in preventing caries in a high caries-risk population. Materials and Methods: Sealant retention and caries status in previously placed sealants on permanent first molars were evaluated in 503 children ages 6 to 8 years at 20 primary schools. A total of 2538 sealants were applied on 876 first permanent molars and evaluated for retention and efficacy in preventing caries from 2002 to 2007. These sealants were placed on occlusal and buccal or palatal surfaces using a standard protocol after isolation with rubber-dam or cotton roll. Caries was scored in teeth in which the sealants were partially or completely lost. Result: 1752 sealants (69%) were examined at the end of the fifth year, with 58.3% of the sealants completely retained, 7.4% partially lost, 19% completely lost, 6.2% resealed and 9.1% restored. Only 3.1% of the previously sealed teeth were carious and 87.8% of previously sealed teeth were caries free. In multivariate analysis, occlusal surfaces were 2.0 times more likely to retain a sealant than were the buccal and palatal pits (p 0.0001). No differences in sealant retention vs caries by arch, or teeth isolated using rubber-dam vs cotton roll were seen. Conclusion: The present study shows the effectiveness of sealants in caries prevention in a multi-operator school sealant programme, and provides evidence supporting their use in such programmes for high-caries populations. Schlagwörter: caries prevention, fissure sealant, retention effectiveness, school sealant programme