Background: Adenocarcinoid is a rare appendiceal tumor that shows histologically both epithelial and neuroendocrine components. We observed that some patients have peritoneal dissemination, which shows adenocarcinoma but lacks the neuroendocrine histology. The objective of the present study was to quantitate these discordant histopathological findings and examine its clinical implication. Methods: Twenty-six patients with peritoneal dissemination from primary appendiceal adenocarcinoid tumors underwent cytoreductive surgery and perioperative intraperitoneal chemotherapy. All resected specimens were individually submitted for histopathological examination and immunostaining with three neuroendocrine markers. The anatomic locations for peritoneal implants were prospectively recorded. The loss of the neuroendocrine tumor type in peritoneal implants was referred to as discordant feature of the adenocarcinoid tumor. Clinical and histopathological prognostic factors for survival were analyzed. Results: In nine of the 26 patients (35%) with a primary adenocarcinoid of the appendix, a loss of the neuroendocrine immunochemical marker was noted in peritoneal implants. Discordant histology between the primary appendiceal adenocarcinoid and the peritoneal implants was statistically significantly associated with an improved survival (p = 0.0262), when patients were treated by cytoreductive surgery and perioperative intraperitoneal chemotherapy. Conclusions: Discordant histology of the primary appendiceal tumor as compared to the peritoneal lesions occurs frequently in patients with adenocarcinoid. An assessment of neuroendocrine markers in both primary and peritoneal lesions may help in the clinical assessment of this group of patients. Small numbers of patients were available for this study and the conclusions are therefore limited.