The aim of this study was to evaluate the incidence of panoramic radiological risk signs related with mandibular third molar extraction, and the relationship between these risk signs and inferior alveolar nerve (IAN) injury after tooth extraction. Cases were defined as 1000 mandibular third molars extracted by surgical approach at Samsung Medical Center during the period from March 2001 to December 2006. Seven radiological risk signs were assessed on the panoramic radiogram by three expert oral surgeons. Clinical demographic data and severity of IAN injury were examined on medical records. Bivariate analyses were completed to assess the relationship between radiological risk signs and IAN injury. The radiological risk signs showed in 381 cases(38.1%). The incidence of each radiological risk signs were; interruption of IAN white line, 152 cases(15.2%); deflected roots, 141 cases(14.1%); darkening root, 119 cases(11.9%); diversion of IAN, 57 cases(5.7%) ; IAN narrowing, 37 cases(3.7%); root narrowing, 17 cases(1.7%); dark and bifid apex, 10 cases(1.0%). The incidence of IAN injury in cases with risk signs were: in the case of any sign, 3.6%; interruption of IAN white line, 2.6%; deflected roots 5.7%; darkening root. 3.4%; diversion of IAN, 5.7%; IAN narrowing, 3.7%; root narrowing, 5.9%; dark and bifid apex, 0%. No IAN injury was showed in 619 cases without risk sign (p<0.05). In conclusion, the presence of panoramic risk signs was associated with an increased risk for IAN injury during mandibular third molar extraction, whereas the absence of risk signs was associated with a minimal risk of nerve injury.