Twenty-two cases of well-differentiated papillary mesothelioma of the peritoneum (WDPMP) are described. Eighteen of the 22 patients were women. The peritoneal tumor was usually multifocal. Many of the tumors appear to be indolent or inactive and for practical purposes are benign. However, a few patients receiving adjuvant therapy have died under circumstances that make it difficult to determine whether the tumor was responsible for the death. It is suggested that adjuvant therapy be withheld from patients with WDPMP, unless there is clear evidence of progression. The cause of these rare tumors is not apparent, although three patients had had possible exposure to asbestos and two were sisters.