The phenotype of the proliferated spindled cells and the histogenesis of inflammatory fibroid polyp (IFP) have been a matter of debate. To clarify the immunohistochemical profile of the main cellular component, we reviewed histologically and studied immunohistochemically 11 cases (12 lesions) of IFP of the stomach. The lesions ranged in size from 0.4 to 3.0 cm (mean, 1.0 cm). All of the IFPs occurred in the antrum. Seven cases (63.6%) were accompanied by carcinoma or adenoma, which in two cases was present in covering epithelium. Grossly, eight IFPs were sessile, and four were polypoid. Microscopically, seven (58.3%) occupied both the mucosa and submucosa. Four (33.3%) were confined to the mucosa, and one was localized only in the submucosa. They were characterized by proliferation of relatively uniform spindled cells, often in a whorl-like arrangement around the blood vessels and glands. Eosinophilic and lymphocytic infiltration of varying degree was always present. The spindled cells in all of the IFPs were diffusely immunoreactive for vimentin and CD34. Seven (58.3%) were focally positive for histiocytic markers, such as KP1 and HAM56. Three (25%) showed focal immunoreactivity for alpha smooth muscle actin. Cytokeratin, desmin, S100 protein, Factor VIII RAG, and Ki-67 were negative in these spindled cells. The principle proliferating cells in IFPs in this study showed variable degrees of histiocytic and myofibroblastic lines of differentiation. Under the influence of some etiologic factors, the primitive, uncommitted spindled mesenchymal cells exhibiting CD34 immunoreactivity around the blood vessels and glands in the lower portion of mucosa and submucosa might play a role in development of IFP.