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Krukenberg tumor, defined as metastatic adenocarcinoma to the ovary containing at least 10% signet ring cells, usually arises from the stomach but can also originate from other sites. We reviewed 17 metastatic breast carcinomas to the ovary with signet ring cells to potentially identify features indicative of mammary origin as opposed to other possible primary sites. The patients ranged from 41 to 76 (mean, 53.6) yr. Fourteen had a prior history of invasive breast carcinoma (invasive ductal carcinoma, 4; invasive lobular carcinoma, 3; adenocarcinoma not otherwise specified, 3; carcinoma with ductal and lobular features, 2; and unspecified carcinoma, 2) and metastases were identified 2 to 284 (mean, 79) mo after the original diagnosis. Three patients had no known history of invasive breast carcinoma: 1 was subsequently diagnosed with invasive lobular carcinoma, 1 had suspicious bilateral breast masses identified on imaging, and 1 was lost to follow-up. Bilateral ovarian metastases were present in 87%, and the tumors ranged from 3.8 to 19 (mean, 8) cm. Microscopically the ovarian architecture was effaced in 71% by discrete tumor lobules separated by striking edema. The tumors exhibited a variety of histologic patterns: nests were most common (88%), followed by cords (82%), diffuse sheets (82%), single cells (71%), small clusters (41%), glands (29%), and follicle-like cysts (12%). Signet ring cells comprised 2% to 70% (mean, 33%) of the tumors, with 14 cases meeting the criteria for Krukenberg tumor. Signet ring cells were most frequently observed within diffuse sheets (71%) and cords (65%). Tumor cells arranged in nests, cords, and diffuse sheets are typical of Krukenberg tumor of breast origin, and the patterns recapitulate those seen in primary breast carcinomas. Features characteristic of gastrointestinal origin, such as extracellular mucin, intestinal-type glands, dirty necrosis, microcysts, and goblet cell carcinoid-like foci, were absent. The overall morphologic picture in cases of ovarian spread of breast cancer with signet ring cells is usually strongly suggestive of mammary origin. The diagnosis can be further supported by the clinical history and immunohistochemical evaluation.