The aim of this study was to examine the effect of a fluoride and xylitol containing toothpaste on the remineralization of human enamel using Quantitative Light-induced Fluorescence (QLF). Human extracted teeth were cut longitudinally into three or four parts, and artificial subsurface lesion windows (2 mm × 3 mm) were created by immersion in demineralizing solution. Each enamel sample (n = 7) was treated for 3 min at 25°C twice a day for consecutive 14 days with the slurry of a silica-based toothpaste 1) without F^- and xylitol (blank), 2) with 500 ppm F^- and 3) with 500 ppm F^- and 5% xylitol toothpaste. In addition, we measured the remineralization ability of a commercially available toothpaste 4) with 500 ppm F^-. The average fluorescence loss F (%) and lesion size (mm²) were determined with QLF. Another variable, delta Q, which was defined as the fluorescence loss integrated over the lesion size (%, mm²), was calculated. The results showed that the combination of 500 ppm F^- and 5% xylitol toothpaste significantly (P < 0.05) recovered both the size and delta Q compared to the other groups. These findings suggested that inclusion of xylitol in fluoride toothpaste might be useful to enhance the remineralization in vivo. (J. Oral Sci. 49, 67-73, 2007)