We studied three intraabdominal tumors that manifested in childhood and were attached to peritoneum, and in which the histologic pattern suggested metastatic tumor of epithelial nature but gave no evidence of a primary neoplasm in the major abdominal organs. Follow-up observation lasted from 1 to 6 years but never disclosed a primary site. Histologic, immunohistochemical, and electronmicroscopic observations indicated a primitive malignant neoplasm of uncertain histogenesis capable of simultaneously expressing epithelial, mesenchymal, and, less consistently, neural phenotypes. In childhood, the possibility of embryonic neoplasm, such as nephroblastoma occurring in atypical sites, is difficult to exclude. Despite the prevailing uncertainty about histogenesis, combined therapy achieved an apparent cure in one of our cases.