The third molar appears to be the most frequently impacted tooth. In most studies, the frequency of mandibular third molar impaction has been found to be 20-30%, with females showing third molar impaction significantly more often than males. During development, the position of the mandibular third molar changes continuously before eruption or impaction. Unless this movement is disturbed, the mandibular third molar will erupt normally into the functional position. Panoramic tomographs are usually taken for clinical examinaion and treatment planning. To determine the timing of surgical intervention, it would be beneficial to determine certain predictive signs about the future eruption from the panoramic radiograph while the tooth is still developing. To estimate whether mandibular third molar are likely to erupt after the age of 20 by determining radiographic feature, we took a study on 25 patients(Average age was 20.2+/-0.7) with 30 impacted mandibular third molars. Panoramic radiograph was taken at age 20 and 25. The results were as follows. 76.9% of the mandibular third molars with initially complete root development fully erupted by the age of 25 years. However, teeth with incomplete root development more often remained unerupted. The mandibular third molar which impacted in only soft tissue or by the occlusal level to the mandibular second molar erupted more often than other impactions. Initially vertical angulated teeth erupted more often than other angulations. There was a statistically significant difference of angulation change between age 20 and age 25 (P<0.05). In the retromolar space/MD width of mandibular third molar ratio, there was a statistically significant difference between the erupted group and the impacted group (P<0.01). It was concluded that we could find the predictive radiographic features to estimate future eruption of mandibular third molars from a panoramic tomograph taken at age 20 years.