Preoperative evaluation of the mandible for invasion by tumor has always been a difficult problem. Various methods have been used, including clinical evaluation, panoramic x-rays, dental films, routine mandible films, bone scans, computed tomographic (CT) scans, and magnetic resonance imaging (MRI) scans. The diagnostic accuracy of these methods has not been totally satisfactory from the clinical standpoint. We compared the diagnostic effectiveness of clinical evaluation, panorex films, and CT scans in 60 patients with carcinoma of the floor of mouth. The relative value of these tests was studied in relation to marginal or segmental mandibulectomy. Our data showed that CT scanning was not very helpful, mainly because of the presence of irregular dental sockets and artifacts. Clinical evaluation was the most accurate, both to determine bone invasion and to decide the type of mandibular resection necessary in association with the primary tumor. Panoramic films were helpful in evaluating the gross extent of mandibular invasion. However, they were not of any help in determining minimal bony invasion or cortical invasion. Even though CT scanning has made a tremendous impact in other areas of head and neck surgery, it is not of much help in making the critical decisions in the type of mandible resection, marginal or segmental, in patients with carcinoma of the floor of mouth. Our experience demonstrates that clinical evaluation is superior in preoperative evaluation of the mandible, and especially in deciding the type of mandible resection.