A 29 year old woman, living in an area with a high level of asbestos exposure, developed the clinical features of peritoneal mesothelioma. The quantitative cytological features differed from those of other mesotheliomas described in the literature in that the tumor cells had a large amount of vacuolated cytoplasm and an extremely low N/C ratio, resulting in a "benign" appearance. The ultrastructural study provided evidence for the production and accumulation of secretory products (mucolipids) by the tumor cells. Treatment with chemotherapy and radiation resulted in temporary remission, lasting for 20 months. However the patient then developed pulmonary involvement of the carcinomatosa a form and pleural tumors. The cytological pattern and the morphometric features of the metastatic floating malignant mesothelial cells in the pleural fluid closely resembled those of the primary peritoneal tumor. This case appears to be an example of secretory peritoneal mesothelioma with a bad prognosis, not withstanding the well-differentiated appearance of the tumor cells.