A critical factor in the pretreatment evaluation of patients with carcinoma of the oral cavity or oropharynx is the presence or absence of bone invasion. A prospective study was performed to compare the sensitivity and specificity of plain radiographs and computed tomography in detecting mandibular invasion by cancer arising in these sites. Forty-three consecutive patients with previously untreated oral cavity or oropharyngeal carcinoma were evaluated preoperatively by intraoral radiographs (dental occlusal views and panoramic radiographs) and CT, and the results were compared with postoperative pathologic findings. Of the 11 cases in which pathologic examination confirmed bone invasion, conventional x-ray films were positive in seven (63.6%). Computed tomography confirmed bone invasion in all 11 (100%) of these patients. In addition, the CT scan was more specific than conventional x-ray films in detecting bone invasion. Based on the findings in this study, we strongly recommend CT as the only radiographic study necessary to evaluate intraoral carcinoma prior to treatment.