The distinction between a primary ovarian mucinous carcinoma or even a borderline mucinous tumor and a metastatic mucinous carcinoma may be difficult. A constellation of clinical, gross pathologic and morphologic features is used in this distinction. One of the most important morphologic features suggesting a metastatic mucinous carcinoma in the ovary is the presence of signet ring cells; these are considered rare in primary ovarian mucinous tumors. In this study, we report 3 primary ovarian mucinous tumors with a component of signet ring cells. The tumors arose in patients aged 27, 55, and 60, were unilateral, confined to the ovary and stage IA. They ranged from 9 to 27 cm; 1 was grossly a multiloculated cystic lesion and 2 were cystic and solid. In one case, the neoplasm had the architecture of a mucinous adenofibroma but had frankly malignant cells lining glands and forming solid aggregates of cells. A second tumor also had the background of an adenofibroma. The third was mostly a mucinous cystadenoma. In one case, endometriosis was present in the same ovary; teratomatous elements were not identified in any case. Immunohistochemistry, performed in 2 cases, showed both to be diffusely positive with CK7 and CA19.9, including the signet ring cells. CK20 was positive in both cases (1 focal; 1 diffuse). Estrogen receptor and CA125 were diffusely positive and carcinoembryonic antigen and CDX2 focally positive in 1 case. Chromogranin and synaptophysin were negative. Investigations to exclude a gastrointestinal neoplasm in 2 cases were negative. Features favoring a primary rather than a metastatic neoplasm are unilateral tumor, low stage, background of adenofibroma or cystadenoma, associated endometriosis in 1 case and an absence of features which are characteristic of secondary mucinous carcinomas in the ovary, such as surface tumor deposits, a nodular growth pattern, and lymphovascular permeation. Immunohistochemistry is of limited value because of overlapping immunophenotype between a primary ovarian mucinous tumor and a metastasis from the stomach, pancreas, biliary tree, appendix, or colorectum, the most likely primary sites for a secondary exhibiting similar features. Our study illustrates that signet ring cells occur rarely in a primary ovarian mucinous tumor; even when conspicuous the features differ from those of the usual Krukenberg tumor. At least some cases of so-called primary Krukenberg tumor may be similar to our cases. However, the designation primary Krukenberg tumor should not be used as, apart from the signet ring cells, a resemblance to a "true" Krukenberg tumor of the secondary type is limited. The tumors should be classified according to the underlying background neoplasm with a notation concerning the signet ring cell component.