The incidence of malignant mesothelioma of the pleura has recently increased in Japan, and ultrastructural and immunohistochemical studies can help in the histopathologic diagnosis. Confronting cisternae consist of dense laminae between the cisternae of rough endoplasmic reticula. Cylindric confronting cisternae have recently been found in patients with acquired immunodeficiency syndrome. The pathologic significance of this unusual structure is still obscure, but it has been proposed that trapped ribosomes on the confronting unit membranes of rough endoplasmic reticula produce the dense laminae. In this study, prominent confronting cisternae were found in more than half the tumor cells, and accumulation of an electrondense fine granular substance surrounded by Golgi vesicles (so-called vesicular rosettes) were noted and found to be continuous with the dense laminae. The nature and origin of the vesicular rosettes are important with regard to the formation and significance of confronting cisternae. Oligocilia have been found in various metaplastic and neoplastic cells and are thought to be nonspecific. There has been only one report of ciliated cells and confronting cisternae in a malignant peritoneal mesothelioma, however, indicating that these unusual cytoplasmic structures might be related to some epithelial-type mesotheliomas.