The authors investigated 29 cases of sinonasal polyps with atypical stromal cells (ASC). The clinicopathologic features of these lesions were of benign inflammatory polyps except for the presence of ASC. Misinterpretation of these cells resulted in contributor diagnosis of sarcoma (rhabdomyosarcoma). Immunohistochemical study of the ASC demonstrated the presence of actin (smooth muscle and muscle specific), KP-1, and vimentin; no reactivity was seen with desmin, myoglobin, S-100 protein, or glial fibrillary acidic protein (GFAP). Unexpectedly, cytokeratin reactivity was identified in more than 75% of the cases analyzed. Ultrastructural analysis revealed that the ASC shared morphologic features in common with fibroblasts and smooth muscle cells. Based on the light microscopic, immunohistochemical, and ultrastructural findings, it was concluded that the ASC represent reactive myofibroblasts and not a neoplastic proliferation. Follow-up data supported this contention indicating the absence of an aggressive biological course. Misinterpretation as a malignant neoplasm might result in unwarranted and unnecessary therapeutic intervention.