Glandular inclusions that appear morphologically benign are occasionally found in lymph nodes as well as in peritoneal and omental biopsies. In patients with gynecologic malignancies, the nature and significance of these mullerian inclusion cysts (MIC) present a diagnostic challenge with regard to whether they are benign and incidental or are related to the coincident tumor for which surgery is being performed. Sixty-two cases of MIC were prospectively identified during a 6-year period. The frequencies were calculated and stratified by lymph node chain distribution, primary tumor site, and primary tumor type. MIC appeared as small cysts lined by a serous (mullerian)-type, cytologically bland, cuboidal to columnar epithelium with a simple architecture. Among 62 women, MIC was found in lymph nodes (27 cases), pelvic peritoneum (19 cases), omentum (16 cases), bowel serosa (9 cases), uterine serosa (8 cases), and parametrial connective tissues (4 cases). Among a set of 417 consecutive cases in which lymphadenectomy was performed, 46 (11%) women had MIC. The MIC involved multiple sites (26 cases in the peritoneum/omentum and 27 in lymph nodes). The primary tumor was in the ovary in 32 of the 46 women with MIC (70%) and of these, 17 were borderline serous (53%). Sixty-two of 6,154 lymph nodes examined contained MIC (1.0%). 3.2% of nodes contained MIC in which the primary tumor arose in the ovary, but only 0.1% with either endometrial or cervical tumors (chi2, p <0.00001). The lymph nodes most often involved by MIC were from para-aortic sites (40%), which reflect the primary drainage route from the ovary. Not uncommonly, neighboring areas in the same lymph node group with MIC disclosed separate foci of obvious metastatic borderline tumor (4 of 10; 40%). In summary, the increased frequency of MIC in lymph nodes sampled for primary ovarian malignancies suggests that MIC in some cases, rather than being benign, incidental inclusions, are more likely bland-appearing forms of metastatic tumor. The preponderance of inclusions occurs with serous ovarian tumors of borderline malignancy, and the inclusions are overrepresented in the lymph nodes that primarily receive drainage from the ovary.