A primary mesenchymal spindle cell tumor with amianthoid fibers was observed in an inguinal lymph node of a 70-year-old woman. With immunohistochemical stains the cells were positive for muscle actin and negative for desmin. Transmission electron microscopy (TEM) showed intracytoplasmic bundles of microfilaments with focal densities, profiles of rough endoplasmic reticulum, and basementlike material indicative of myofibroblastic and/or smooth-muscle cell differentiation. At scanning electron microscopy (SEM) the tumor was organized into slitlike spaces containing red blood cells. Amianthoid fibers showed a strong positivity for actin at their edge with a central negative core; by TEM they appeared to be formed of 200 nm thick crystallized collagen fibers surrounded by actin microfilaments arranged in a palisading fashion. The immunohistochemical and ultrastructural findings of the proliferating myoid cells suggest a possible derivation from smooth-muscle vascular cells. Collagen fiber aggregation and actin filament dismission might be secondary to anoxia due to vascular obliteration or disruption.