UNLABELLED: The efficacy of fissure sealants (FS) in controlling occlusal caries is well documented. However, several factors can potentially modify the caries-preventive effect of FS and their failure rate in real life might be higher than in controlled experimental settings. The aim of this study was to prospectively examine failures and risk factors in sealed first permanent molars (FPMs) with sound occlusal surfaces or initial caries lesions of children in a practice-based setting in public dental service (PDS) in Norway. METHODS: Children aged 6 to 10 years considered at high caries risk (D3MFT/d3mft >
0) were recruited by dentists and dental hygienists during routine examination at local dental clinics in PDS. After the application, resin-based FS were maintained according to clinicians' conventional procedures. Occlusal surface/FS status and the following treatment were registered at 6- , 12-, 24- and 36-months. The outcome was categorized into success, minor failure, and failure of sealed FPM based on a combination of diagnosis of occlusal surface/FS status and the following treatment at each follow-up. Univariable and multivariable Cox survival analyses were used to investigate potential risk factors for failure. RESULTS: Out of 409 participants, 4% (16) were lost to follow-up. Of 393 FPMs, 72% (284) were evaluated as success, 9% (34) as minor failure, and 19% (75) failed during the 36-months of this study period: 13% (51) received FS reapplication and 6% developed dentine caries or were restored. One third of FS were additionally treated with fluoride varnish (FV) at least once during the follow-up. Occlusal surface with caries grade 1-2 (vs intact) increased (adHR 1.9, 95% CI 1.1-3.1), while FV application over FS decreased hazards (adHR 0.4, 95% CI 0.3-0.7) for failure. The clinic's county was associated with failure. CONCLUSION: After 3-year follow-up, nearly one out of five sealed FPMs failed, i.e., had to be resealed, developed dentine caries, or were restored. Initial caries lesion on occlusal surface increased while FV application over FS reduced risk for failure. Moreover, regional differences in sealed FPM failures were observed.