The aim of this clinical study was to evaluate the diagnostic outcome of the detection of smooth surface lesions by the Quantitative Light-induced Fluorescence (QLF) method (mirror version) in comparison with visual inspection (VI). 34 fifteen-year-old adolescents were involved in the study. After professional tooth-cleaning 918 buccal and 917 lingual surfaces of the subjects were examined visually with aid of compressed air and magnifying glass (3.5x). Fluorescence images of the smooth surfaces were captured by a CCD camera after excitation with blue light (lmax. 404 nm). A computer programme (Inspektor QLF 2.00) was used to display, store and analyse the images. QLF images were analysed blind by two examiners, presence or absence of a lesion was not marked on the fluorescence images. Average fluorescence loss (DF) of the lesion, the area (A) of the lesion (mm2) and DQ (DF * A) were determined. After exclusion of all surfaces scored as sound, filled or with frank lesions by both methods 17.8% of the buccal/lingual initial enamel lesions (n=489) were detected by VI, 53.8% by VI + QLF and 28.4% by QLF, respectively. There was a significant difference (Mann-Whitney U-test) of the parameters A, DF and DQ between smooth surface lesions recorded by VI + QLF and QLF only. QLF was able to detect smaller enamel lesions with smaller fluorescence loss than VI + QLF. It was concluded that QLF is a sensitive method suitable for quantification of visual undetected incipient caries lesions. Lesion detection by QLF was limited in cases of partially erupted surfaces and of small focal depth on lingual surfaces as well as in patients with poor oral hygiene associated with reduced surface size caused by gingivitis. Keywords: caries detection, optical methods, quantitative light induced fluorescence