BACKGROUND. Malignant mixed mesodermal tumors (malignant mixed Mullerian tumors [MMMT]) occur rarely in extragenital sites. METHODS. The authors analyzed the clinical, pathologic, and immunohistochemical features of three cases of primary MMMT of the female peritoneum. RESULTS. The neoplasms occurred in 60-, 64- and 84-year-old women and arose from pelvic peritoneum. Two patients died with disseminated disease 8 and 24 months postoperatively. The third died of cardiac failure 12 months postoperatively with questionable metastatic disease. Microscopically, two tumors were of the heterologous type, containing foci of rhabdomyosarcomatous (case 1) and chondrosarcomatous (case 3) differentiation. Immunohistochemically, coexpression of keratin and vimentin was observed focally in both carcinomatous and sarcomatous components in all three neoplasms, whereas coexpression of low molecular weight cytokeratin, vimentin and actin was observed focally in case 2. Rhabdomyosarcomatous areas were positive with desmin and actin, and chondrosarcomatous areas for S-100 protein. Both epithelial and mesenchymal components were positive for alpha-1 antichymotrypsin in all cases. CONCLUSIONS. On the basis of the present cases and a review of 15 reports from the literature, primary MMMT of the female peritoneum proved to be a rare but highly malignant neoplasm occurring in elderly postmenopausal women. Of 15 patients with available follow-up, 12 died with disease, mostly within 1 year, regardless of the initial tumor stage, histology (homologous versus heterologous MMMT) or treatments attempted. The tumor developed within pelvic peritoneum in half the cases. Histogenetically, peritoneal MMMT are thought to represent "metaplastic" carcinomas originating from the secondary Mullerian system