It may be difficult to distinguish ovarian involvement by a low-grade appendiceal mucinous neoplasm (LAMN) from a primary gastrointestinal-type primary borderline (proliferative) ovarian tumor (IBMT) or an ovarian mucinous tumor arising within a teratoma, particularly when the latter is associated with mucinous ascites/pseudomyxoma peritonei. We noted that LAMNs involving the ovaries show 2 distinctive histologic features, "scalloped" glands and subepthelial stromal clefts, whereas IBMTs more often are associated with reactive cellular stroma and histiocyte aggregates (mucin granulomas). The frequency of these features was investigated in 18 LAMNs (16 with pseudomyxoma peritonei), 18 primary IBMTs, and 6 teratoma-associated mucinous tumors (selected on the basis of associated pseudomyxoma peritonei). Scalloped glands and subepithelial clefts were identified in 17 and 16 LAMNs, respectively, and in 3 and 7 IBMTs, respectively. Conversely, reactive stroma and histiocyte aggregates were present in 2 and 0 LAMNs, respectively, and in 11 and 10 IBMTs, respectively. LAMNs were often bilateral (12/18 cases) and they more frequently showed mucin dissection of the ovarian stroma and tall mucin-rich (hypermucinous) epithelial cells compared with IBMTs. Our findings suggest that scalloped glands, subepithelial clefts, cellular stroma, and histiocyte aggregates may be useful additional morphologic parameters to help distinguish these tumor types. However, teratoma-associated mucinous neoplasms can show identical histologic features to those of LAMNs involving the ovary, and therefore accurate diagnosis of such cases requires careful macroscopic and microscopic examinations of the ovaries together with complete histologic assessment of the appendix. © 2013 International Society of Gynecological Pathologists.