A diffuse, well-differentiated, malignant peritoneal mesothelioma (MPM) developed in a nine-year-old girl. She received limited chemotherapy and radiation therapy and is alive and well without clinical evidence of disease 109 months after diagnosis. The neoplastic cells stained immunohistochemically for cytokeratin and epithelial membrane antigen but were unreactive with B72.3, anti-carcinoembryonic antigen, and anti-Leu-M1. Ultrastructurally, the tumor cells had abundant desmosomes, numerous tonofilament bundles, and variable-length microvilli. These findings confirm the mesothelial nature of the cells. Features consistent with malignancy included DNA aneuploidy by flow cytometric analysis and diffuse peritoneal involvement. The three previously described survivors with MPM were also premenarchal girls. Some MPMs in premenarchal girls have an indolent biologic behavior similar to that of low-grade peritoneal serous neoplasia or well-differentiated papillary mesothelioma in adult women.