Desmoplastic small round cell tumor (DSRCT) is a rare and highly aggressive tumor usually involving the peritoneum. It occurs more commonly in young males and is characterized by distinctive clinical, histologic, and immunophenotypic features. The histogenesis of DSRCT remains unknown. Coexpression of epithelial, mesenchymal, and neural antigens in the same cell provides evidence that DSRCT may arise from a primitive pluripotent stem cell with divergent differentiation. Recently, according to cytogenetic studies, some authors have proposed that the divergent differentiation of DSRCT may be the result of the fusion of Ewing's sarcoma gene and Wilms' tumor suppressor gene. Clinically, an elevated serum CA 125 concentration is found in some patients with DSRCT. We present the case of a 29-year-old man with diffuse intra-abdominal DSRCT and elevated serum CA 125 concentration and briefly review the relevant literature.