Pseudomyxoma peritonei is a clinical term for gelatinous ascites, usually secondary to an appendiceal tumor. The pathologic classification of pseudomyxoma peritonei and its associated appendiceal tumors has been plagued with controversy and confusing terminology. In an effort to clarify this, we reviewed the pathology of 101 patients, all treated at our institution from 1993 to 2005, with pseudomyxoma peritonei of appendiceal origin. All patients were uniformly treated with our standardized protocol. This is the largest pathologic series solely devoted to appendiceal neoplasia with gelatinous ascites. The cases were assigned, according to previously published criteria, to the categories of disseminated peritoneal adenomucinosis (DPAM), peritoneal mucinous carcinomatosis (PMCA), or PMCA with intermediate (well differentiated) features (PMCA-I), with the exception that any case with a signet-ring cell component was considered as PMCA and not PMCA-I. By histologic category, 58 patients had DPAM, 23 were PMCA, and 20 were PMCA-I.One-year, 3-year, and 5-year survival outcomes were not significantly different between DPAM and PMCA-I. DPAM and PMCA-I also exhibited a roughly equal incidence of parenchymal (beyond the serosa) organ invasion. Survival outcomes were significantly worse for PMCA, compared with PMCA-I and DPAM. After reviewing our data and the literature, mucinous carcinoma peritonei-low grade was applied to the low-grade histology of pseudomyxoma peritonei, including those cases referred to by some as DPAM in the same category as PMCA-I. Cases that are moderately differentiated to poorly differentiated are classified as mucinous carcinoma peritonei-high grade.