Maintenance of sealant integrity is generally equated to clinical effectiveness. Reported rates of sealant loss in children averaged at nearly 50% in Germany. This raises the question whether partial sealant loss needs replacement, repair or monitoring. Aim of this study was to test the potential of light-induced fluorescence (LF) for judging the quality of fissure sealants compared with clinical assessment. After plaque disclosing (Miraton, Hager) and professional tooth-cleaning 280 occlusal surfaces of 35 fifteen-year-olds were examined visually with aid of compressed air and magnifying glass (3.5x). Sealants were scored clinically as (1) present with marginal integrity and (2) partial loss and detectable margin with or without a discoloured fissure. Fluorescence images of the occlusal surfaces were captured by QLF\\clin (light source: 35 W Xenon microdischarge arc lamp). Images were stored with an Inspektor QLF 2.00 programme. LF images were assessed by two examiners. Additional to the clinical scoring criteria (1, 2) presence or absence of red plaque fluorescence, presumably originates from porphyrins produced by anaerobic bacteria [König K, et al.: SPIE 1993;2080:170-180], was recorded. A total of 89 sealants was scored. 37 sealants were clinically intact and 52 revealed a failure. In 52% of all cases with partial loss discoloured fissures were observed. LF images showed red fluorescence at the margin of 10 sealants clinically scored as intact (1). Partial sealant loss was combined with red plaque fluorescence in the exposed undiscoloured fissure in 17 of 26 cases. The relation of sealant loss with brown discoloured fissures with or without red fluorescence was 10 to 15. It was concluded that LF enables a sensitive assessment of sealants quality. It is debatable whether the presence of bacteria at a sealant margin would automatically justify sealant replacement. Keywords: QLF, fissure sealants, quality assessment, diagnostics