The accuracy of preoperative assessment in determining invasion of the mandible by intraoral squamous cell carcinoma was analyzed in 48 patients who underwent mandibulectomy, and the results correlated with the histopathological reports of the resected specimens. Only 50% of the patients underwent the "ideal" surgery based primarily on clinical judgement, whereas 10 patients in the series were significantly undertreated. Clinical judgement and routine preoperative x-rays are accurate in cases where there is gross involvement of the mandible (17 of 19) but are significantly less successful in determining early bone invasion, invasion of the periosteum, or periosteal new bone formation. In such cases (26 of 48), a technetium-99m bone scan provides additional information. A grading system for reporting orthopantomographics (OPTs) and bone scans has been developed and utilized to form a reference grid to determine the optimum extent of mandibular surgery. The results show that using this protocol, unnecessary mandibular surgery may be reduced and inadequate surgical excision avoided.