The authors conducted a study to evaluate the predictive value of five radiographic markers on the panoramic radiograph, or PR, to point out the relationship between the mandibular canal and the impacted third molar. The authors evaluated the accuracy of the radiographic markers by comparing the PR with an axial computed tomographic, or CT, scan. They identified a sample of 73 third molars that showed a close relationship between the tooth roots and the mandibular canal on the PR, and then classified them on the basis of five radiographic markers. They also detected contact between the third molar and the mandibular canal on the CT scan. The distribution of the five radiographic markers was as follows: 37 teeth exhibited increased radiolucency, 13 exhibited superimposition, 14 exhibited interruption of the radiopaque border, 14 exhibited narrowing of the canal and seven exhibited diversion of the canal. In 11 cases, two or more markers were recognizable. The predictive values of a positive test result were as follows: increased radiolucency, 73 percent; superimposition, 38.5 percent; interruption of the radiopaque border, 71.4 percent; narrowing, 78.6 percent; and diversion, 100 percent. The authors detected contact in all of the cases that exhibited two or more markers. Increased radiolucency, narrowing and interruption of the radiopaque border, as well as the concomitant presence of two or more radiographic markers, on the PR were highly predictive of contact between the third molar and the mandibular canal. An axial CT scan probably is indicated in such cases. The results of this study may lead to some guidelines for oral surgeons evaluating whether to obtain an axial CT scan for further investigation after examining an impacted mandibular third molar via PR.