A case of a benign cystic peritoneal mesothelioma is reported. A 25-year-old woman presented a 8-year history of recurrent tumor and five operations because of that. On the basis of this case and of 18 cases proved by electron microscopy reported by others, the clinicopathological features and the problems of differential diagnosis are discussed. The ultrastructural examination is essential for the diagnosis of cystic mesothelioma.