Three female patients with a diffuse well-differentiated papillary mesothelioma of the peritoneum are presented. They did not have a history of exposure to asbestos. The peritoneal biopsies were studied extensively, including electron microscopy, nuclear morphometry and DNA ploidy analysis. The results from these more sophisticated investigations confirmed the mesothelial origin and further characterised these lesions. One of the 3 patients has continuing elevated serum CA-125 levels, which increased transiently during and after pregnancy. All 3 patients have done well without therapy, 2 patients being alive and non-symptomatic 6 and 7 years after the initial diagnosis. It is important to distinguish this disorder from the malignant diseases of the peritoneum and the ovary. In view of the indolent course of this subtype of mesothelioma, avoidance of treatment is justified unless there is evidence of progressive disease.