Quantitative laser fluorescence has been reported as a useful method for the non-destructive in vitro and in vivo diagnosis of early enamel caries. A portable system for intraoral use has been developed with a new light source and filter system replacing the laser light to facilitate clinical application. This new device was validated with microradiographic and chemical analyses for assessment of mineral changes in enamel during lesion formation and remineralization in vitro and compared with the laser light equipment. A significant correlation was found between fluorescence changes and mineral loss: r = 0.79 (laser system) and r = 0.84 (portable lamp system). The correlation between the two fluorescence methods was r = 0.93. The portable fluorescence device seemed to be a promising new tool for reproducible and sensitive assessment of the severity of incipient enamel lesions.