The ability of an en-face optical coherence tomography (OCT) system to detect and quantitatively monitor the demineralization of teeth to produce enamel caries was determined and validated with an established method of quantifying demineralization, transverse microradiography (TMR). Factors that might affect the use of OCT to detect and quantify caries were also examined. We used an OCT system which can collect A-scans, B-scans (longitudinal images) and C-scans (en-face images). Development of dental caries was quantitatively monitored every 24 h for 3 days. Caries lesions were shown as volumes of reduced reflectivity in both the transversal and longitudinal images. It was observed that the reflectivity of tooth tissue decreased with increasing demineralization time. A linear correlation was observed between the mineral loss in enamel measured by TMR and the percentage reflectivity loss measured by OCT. Neither the presence of saliva, plague, Dacron gauze, plaque/Dacron gauze, nor lesion staining nor the level of ambient lighting significantly affected OCT detection and analysis of an incipient caries. We concluded that OCT could detect early enamel caries and quantitatively monitor the demineralization of the tissue. The reflectivity loss in tooth tissue during demineralization, measured by OCT, could be related to the amount of mineral lost during the demineralization.