The proposed origin from the adult gingival epithelium for peripheral ameloblastoma is difficult to prove and still remains unresolved. This report details a small fibrous polyp on the maxillary edentulous ridge of a 73-year-old woman, in which the basal layer of covering squamous epithelium dipped down toward the lamina propria over a wide area. Basal cells of the downgrowth were ameloblastic in type, and bulbous parts assumed an appearance of the bud-shaped enamel organ. Multiple sectioning failed to detect any epithelial nests in the submucosa. In addition to the morphologic overlap, early ameloblastoma epithelia shared antigenic properties with tumor cells of representative control examples of peripheral ameloblastoma. This is the first description of an in situ lesion of peripheral ameloblastoma, to our knowledge, and its multifocal ameloblastoma changes can be accepted as a direct proof of origin from the surface epithelium.