Intraosseous schwannoma, a rare benign neoplasm, has a characteristic radiographic appearance. The histologic features, although similar to those of soft tissue schwannomas, may be obscure when the lesions are highly cellular and the Antoni Types A and B patterns are subtle. Four additional cases of intraosseous schwannoma are presented, with ultrastructural studies from two tumors, one of which was highly cellular and presented a diagnostic problem. The ultrastructural features of intraosseous schwannomas have not been documented previously; however, the findings are essentially the same as those observed in soft tissue schwannomas. The radiographic, histologic, and ultrastructural findings in the cases reported are discussed in relation to published data for intraosseous and soft tissue schwannomas. The authors propose that the comparatively high frequency of mandibular involvement by intraosseous schwannoma may be related to the fact that schwannomas, in general, arise most frequently in the head and neck regions. Furthermore, since schwannomas arise mainly in association with sensory nerves, probably the dearth of such fibers within bone accounts for the rarity of intraosseous schwannoma.