The objective of this prospective study is to assess the impact of magnetic resonance imaging (MRI) and computed tomography (CT) as compared to physical examination in the choice of type of surgery for advanced intraoral cancers (with or without resection of the mandibula). From 1990 to 1993, we operated on 21 intraoral malignant tumors with segmental resection of the mandibula followed by a histological examination. The preoperative evaluation consisted of an MRI (n = 8), a CT (n = 8) or both (n = 5). MRI suspected an infiltration of the bone in 9 cases, CT in 4 and physical examination in 16. This was histologically confirmed in 6 of the 21 patients only. MRI and CT both have a high sensitivity, as does physical examination, but neither have a good specificity (physical examination: 5 true positive, 4 true negative, 11 false positive, 1 false negative; MRI: 4 true positive, 4 true negative, 5 false positive, 0 false negative; CT: 3 true positive, 7 true negative, 3 false positive, 0 false negative). In conclusion, the decision of a mandibular resection can only be taken after a careful physical examination, including palpation under general anesthesia in a fully relaxed patient. This is best accomplished during the pretherapy bronchoesophagoscopy, routinely performed for the detection of synchronous second primary tumors using toluidin blue as a vital staining method. If this initial evaluation gives a suspicion of a massive infiltration of the mandible, an MRI, rather than a CT, should be performed to determine the extent of the resection because of a high rate of artefacts with CT.