Three cases of infantile digital fibromatosis were studied by electron microscopy and immunohistochemistry. The tumor was made up equally of myofibroblasts containing long narrow bundles of microfilaments with dense bodies. Another striking feature was the intracytoplasmic electron-dense inclusion. The inclusions were constituted of packed fibrils similar to those of the long bundles. At the periphery of these inclusions were seen prominent felt-like filamentous structures; the diameter varied from 5-12 nm. The 10 nm filaments were seen most abundantly in the cytoplasm surrounding the inclusions, but sometimes they were also found within the inclusions themselves, mainly at their periphery. These findings suggest that the inclusions may represent an abnormal accumulation of cytoskeletal proteins. On the other hand, various kinds of immunohistochemical study showed that actin, actomyosin, myosin, and vimentin were distinctly observed in cytoplasms of tumor cells. They were located fringing the intracytoplasmic inclusions, but the inclusion themselves showed negative findings, thus indicating a hollow-like staining pattern. These data can be synthesized as follows. The inclusion results from an accumulation of cytoskeletal proteins (actin-myosin complex and vimentin, at least), and antigenicity seems to be lost by for a certain mechanism or in the degenerative process.