The medial cortical surface of the mandible can be involved by tumour infiltration from the floor of the mouth. A detailed study of spread via accessory foramina through the edentulous alveolar crest has been previously undertaken, but no similar study has been carried out for the medial surface. In order to gain further appreciation of the mode of tumour spread, a study of the number and distribution of accessory foramina on the medial mandibular surface was performed on 89 mandibles. The number of foramina varied greatly from specimen to specimen. In the ascending ramus above the inferior dental foramen, 3 mandibles showed no foramina; the condylar section possessed the greatest proportion followed by the sigmoid and the coronoid. On the rest of the medial surface below the inferior dental foramen, all specimens showed at least 1 accessory foramen; the greatest concentration was in the middle third along the path of the inferior dental canal, followed by the upper third and the lower third section. Accessory foramina were repeatedly present at certain dedicated sites. The medial facing wall of the inferior dental foramen was found to be the commonest dedicated site (98.3%) followed by foramina on either side of the genial tubercles (71.9%), the digastric fossa (71.9%) and the median foramen above the genial tubercles (64%). The findings of this study are in keeping with the current observation that the lower border is least commonly involved in tumour spread. In view of the presence of accessory foramina along the inferior dental canal and especially on the medial facing wall of the inferior dental foramen, it is imperative to preclude tumour spread in this region prior to undertaking the conservative rim resection procedure. Medial to the symphysis the alveolar mucosa dips down almost to the level of the dedicated foramina in the vicinity of the genial tubercles. As a general rule the attached muscle forms a barrier to tumour spread except in the later stages, however, in irradiated mandibles resistance to spread has been previously reported to be diminished. Under these circumstances, it is possible that the numerous accessory foramina reported in this study could facilitate a direct pathway into the cancellous bone.