The radiologic and histologic features of mandibular invasion, and its clinical implications, are considered in a retrospective series of 111 patients with squamous carcinomas of the oral cavity and oropharynx treated by composite resection. Eighty percent of the entire group had either recurrent or advanced (T3, T4) local disease, and 33 patients (30%) had histologic evidence of mandibular invasion by tumor. Preoperative radiologic assessment was unreliable in cases in which infiltrating tumor was confined to the periosteum and superficial cortex—44% false negatives. The extent of bone invasion was found to correlate with the size of the tumor, but not with its histologic grade. The mandibular periosteum was not seen as a morphologically discrete “barrier” and infiltration occurred at various points along the mandibular body, mainly related to the course of the inferior dental canal. The gross and microscopic patterns of bone invasion appeared to be similar in irradiated and nonirradiated resections. The incidence and pattern of recurrent disease following composite resection was the same in the groups with and without mandibular invasion: in each group half the patients were dead from disease and one third alive and free of disease at 2 years. Mandibular invasion alone did not appear to influence prognosis in this series.