The use of ultrasound for the diagnosis of natural caries lesions with different depths on flat surfaces of permanent molar teeth was studied. A total of 200 freshly extracted permanent molar teeth with natural caries lesions were included. After evaluation with International Caries and Detection Assessment System II (ICDAS II), mineral loss and lesions in the teeth were detected with a fluorescence device, FluoreCam and lesion depths were measured using ultrasound. The teeth were sectioned and examined histologically using a microscope. The intraclass correlation coefficient was 0.97 for the ultrasound system, 0.65 for the ICDAS II classification and 0.59 for FluoreCam (p < 0.001 for all). With an increase in ICDAS II scores, mean ultrasound values, FluoreCam-Size and microscope measurements increased numerically, while FluoreCam-Intensity and FluoreCam-Impact measurements inversely decreased (p < 0.001). There was a high level of agreement between histologic diagnosis and ultrasound. Ultrasound and FluoreCam can be used to detect enamel caries on flat surfaces. (E-mail: firstname.lastname@example.org) (c) 2020 World Federation for Ultrasound in Medicine & Biology. All rights reserved.