A case of rapidly fatal small cell sarcoma of kidney in a young woman is described. Histologic, immunohistochemical, and ultrastructural studies demonstrated some features not reported previously in the literature, such as the arrangement of vimentin intermediate filaments in small irregular masses and the presence of various types of cytoplasmic projections. These findings, together with the observation of rare neoplastic elements with immunophenotypic and subcellular aspects of histiocytic and myofibroblastic differentiation, suggest that this tumor should be included in a group of small cell sarcomas of kidney lacking features of differentiation and should be considered a new variant occurring in adulthood.