External resorption is often first detected radiographically. Early detection can lead to timely intervention and improvement of treatment outcome. The purpose of this study was to determine the minimal radicular defect size in maxillary anterior teeth that is radiographically detectable. Six teeth were selected in a cadaver maxilla (#6-11) and extracted. The teeth were then replanted in the maxilla, and three horizontally angled radiographs (0°, 30° from the mesial, and 30° from the distal) were exposed as a baseline. Then, a sequence of tooth removal, placement of a 0.1-mm deep defect in the interproximal and midroot surface, tooth replantation, and radiograph exposure was begun and repeated eight times. Each time the defect depth was increased by 0.1 mm. Ten clinicians interpreted the subsequent radiographs to determine if they visualized a defect on each tooth and the location of the defect. The evaluators' ability to detect experimental lesions according to the depth of the defect was shown to be statistically significant for both tooth type and location of the radicular defect (analysis of variance, p < 0.05). The average size of defect needed for radiographic visualization was as follows: central incisor (0.28 mm on the interproximal [IP] and 0.74 mm on the midroot [MR] [palatal], lateral incisor (0.39 mm on the IP and 0.55 mm on the MR); and canine [0.45 mm on the IP and 0.71 mm on the MR). Based on the results of this study, the minimal defect size detected was 0.28 mm to 0.74 mm depending on defect location and tooth selected.