A benign pelvic soft tissue tumor from a 50 year old woman was examined by immunohistochemistry and electron microscopy. The tumor cells had abundant eosinophilic cytoplasm with a hyaline appearance, which was filled with large aggregates of intermediate‐sized filaments (IF). The cells were positively immunostained by antibodies against cytokeratin, vimentin, desmin, glial fibrillary acidic protein, and neurofilament proteins. This case represents an extreme example of the simultaneous expression of IF by neoplastic cells, and exemplifies the limited applicability of immunohistochemical detection of IF antigens for pathological diagnosis of neoplasms. Acta Pathol Jpn 41: 65–72, 1