OBJECTIVE The purpose of this study was to determine the diagnostic accuracy of CT in detecting mandibular invasion by squamous cell carcinoma of the oral cavity. METHODS Forty-nine patients who had squamous cell carcinoma of the oral cavity that was clinically fixed to the mandible were treated with mandibulectomy. All patients underwent contrast-enhanced CT (contiguous 3-mm-thick sections) through the primary site before surgery. All studies were reconstructed with bone algorithm. These studies were retrospectively reviewed by a neuroradiologist for evidence of mandibular invasion. The imaging results were compared with the histologic findings in all cases. RESULTS CT correctly revealed 25 of 26 cases with mandibular invasion. CT correctly excluded mandibular invasion in 20 of 23 cases without invasion. The diagnostic accuracy of CT for detecting mandibular invasion was as follows: sensitivity, 96%; specificity, 87%; positive predictive value, 89%; and negative predictive value, 95%. CONCLUSIONS Thin-section (3-mm) CT reconstructed with bone algorithm is an accurate technique to detect mandibular involvement by squamous cell carcinoma of the oral cavity.