The well-documented but rare primary papillary serous peritoneal tumors are difficult problems for the pathologist and the clinician. Because of their unusual location, these tumors are often classified as mesothelioma or advanced ovarian carcinoma. In this study, we report the clinicopathologic features of eight primary peritoneal serous papillary tumors and compare their histologic and ultrastructural features to 12 serous ovarian tumors and 16 epithelial mesotheliomas (two peritoneal and 14 pleural). The eight peritoneal serous papillary tumors occurred in women aged 19 to 75 years; two were serous tumors of low malignant potential (borderline) and six were serous carcinomas. The tumors were located in the mesosalpinx, left pelvis, omentum, and/or surface of the ovary. The two patients with borderline neoplasms had long disease-free survival (11 years and 20 years), while three of the four patients with carcinoma with more than 1 year of follow-up died of disease. The peritoneal serous papillary tumors were morphologically identical to serous ovarian tumors of equivalent grade. Well-differentiated papillary structures with distinct fibrovascular cores and one or several layers of columnar, crowded cells, dense overlapping nuclei with a long axis perpendicular to the surface of the papillary cores, and numerous psammoma bodies were features of the peritoneal and ovarian serous tumors. In contrast, the tubulo-alveolar, solid, or poorly defined papillary structures lined by well-spaced polygonal to cuboidal cells with abundant cytoplasm, absence of nuclear polarity, and infrequent psammoma bodies characterized the mesotheliomas. Epithelial mucin and carcinoembryonic antigen (CEA) immunoreactivity, when present, supported a diagnosis of serous tumor in these generally mucin-poor and CEA-negative neoplasms. Ultrastructurally, the cells of serous tumors had slender, straight microvilli of variable length interspersed with or without cilia, while the nonciliated cells of mesothelioma had long, exuberant, wavy microvilli. The morphologic and clinical features of the peritoneal papillary serous tumors are distinctive enough to warrant their separation from mesotheliomas.