Desmoplastic small cell tumor (DSCT) is a distinctive, poorly understood malignant tumor with a complex phenotype. The main features of the 46 cases presented here include a mean age of 20.8 years (range 7-48), male predominance (37 M:9 F), primary intraabdominal location (except for 2 cases), characteristic histologic pattern consisting of tumor cell nests surrounded by a cellular desmoplastic stroma, immunohistochemical reactivity for markers of epithelial, muscle and neural differentiation, and association with a unique cytogenetic abnormality. These findings suggest that this tumor is a distinct neoplastic process and that it probably belongs to the family of primitive small round cell developmental tumors. It is proposed that DSCT arises during development from a progenitor cell with potential for multiphenotypic differentiation.