A case of Castleman's disease occurring in the buccal mucosa is described. An 84-year-old woman noticed that a mass in the left buccal mucosa that had been present for half a year. Computed tomography revealed a well-demarcated submucosal tumor, measuring 4.0 x 3.0 x 2.0 cm. The patient received no treatment at this time, and continued growth of the mass was observed. After incisional biopsy, the lesion was surgically removed. Histologically, the tumor consisted of an enlarged lymph node with conspicuous lymph follicles, in which vascular channels and deposits of eosinophilic material were noted. Laboratory examination showed an increase of serum antibody level of cytomegalovirus but of no other viruses. The patient was followed up for 1(1/2) years, with no clinical evidence of recurrence. This is the first report of Castleman's disease presenting in an oral site.