A tumor in the skin of a 42-year-old man was analyzed by light and electron microscopic study and immunohistochemistry. The tumor cells were large and contained eosinophilic, periodic acid-Schiff (PAS)-positive inclusions in the cytoplasm. Immunohistochemically, the neoplasm was positive for intermediate filaments, cytokeratin, vimentin, desmin, and alpha-1-antichimotrypsin, and negative for S-100 and leukocyte common antigen (LCA). Ultrastructurally, the cytoplasm and cellular processes of the cells were inundated with intermediate filaments, some of which were tightly bundled. Junctional complexes and secretory granules were absent. These features suggest a rhabdoid tumor: a malignant tumor of uncertain origin.