Using a cystic lymphangioma of the greater omentum and a benign cystic mesothelioma as examples, the distinguishing characteristics of cystic peritoneal tumors are shown, using light microscopic and ultrastructural findings. A benign papillary mesothelioma is used for comparison. The cellular structures and growth rate of the mesotheliomas are contrasted with the tissue reactions which are typical for irritated serosa. The diffusely growing papillary mesothelioma is a very rare tumor, the cells of which are similar to normal serosa cells, but also show characteristics of other benign or malignant mesothelial tumors and of reactive proliferated mesothelial cells. The diffusely growing benign cystic mesothelioma has only been described in isolated cases and is characterized by cysts grouped in acini with mainly flat, localized cuboidal cell lining. The histochemical properties and cytological findings correspond closely to those of the papillary tumor or normal peritoneal lining cells. In contrast, the cystic lymphangioma probably represents a congenital defect with a slow growth rate. The structure is characterized by a sponge-like arrangement of smooth-walled cysts, in the walls of which smooth muscle cells and lymph follicles are embedded. The endothelium is also flat and ultrastructurally resembles that of lymph vessels.