The surgical removal of an impacted mandibular third molar may result in damage to the inferior alveolar nerve and may cause disabling anaesthesia of the lip; anaesthesia of the lower gingivae and anterior teeth may also result. Assessing the likelihood of injury depends to a great extent on preoperative radiographic examination. Seven radiological diagnostic signs have been mentioned in the literature; the reliability of these signs as predictors of likely nerve injury have been evaluated through retrospective and prospective surveys. Three signs were found to be significantly related to nerve injury and a further two were probably important clinically.