Radial forearm skin flaps transplanted into the oral cavity in patients undergoing treatment for intra-oral tumors frequently acquire a gross appearance resembling that of buccal mucosa. This article reports studies of biopsies of the flaps in 53 patients, of which 21 showed changes at some stage in the follow-up period. The project aims to determine whether 1) the change in the epithelium to a mucosal form is genuine or reactive, 2) a creeping replacement of epidermis results from the surrounding buccal epithelium, 3) alterations in the epidermis could be associated with changes in the dermis, 4) the change is caused by radiotherapy. Examination of the biopsies by resin histology and transmission electron microscopy shows that the changes may be reversible: it suggests that they are reactive in nature, and related to the degree of dermal inflammation, rather than to the effect of radiotherapy. The dermis persists as such and is not replaced by buccal lamina propria.