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Background: Seromucinous borderline tumor (SMBT) is a rare neoplasm which was newly defined in the 2014 WHO classification. Although the clinical and histopathological characteristics of SMBT have been well described, its cytological characteristics have not. We recently experienced 2 cases of SMBT which were defined by cytological findings of ascites. Case Presentation: Case 1 was a 65-year-old Japanese woman. Preoperative imaging studies revealed abundant ascites and a cystic tumor, with a solid component measuring 13 cm on the left ovary. Radical surgery was performed during the intraoperative diagnosis of ovarian borderline tumor, made by histological examinations of frozen tumor sections. The cytological smears of preoperatively and intraoperatively collected ascites showed many atypical cells resembling reactive mesothelial cells. Alcian-blue staining of cell block sections revealed intracytoplasmic mucin, and the results of immunohistochemistry were consistent with SMBT. The final pathological diagnosis of tumor was SMBT. Case 2 was a 28-year-old Japanese woman. Preoperative imaging studies revealed a small amount of ascites and cystic tumors with solid components in the bilateral ovaries. She initially underwent fertility preservation surgery. SMBT was suspected by cytological examination of smears of intraoperatively collected ascites and the findings of cell block. She underwent additional radical surgery based on a final pathological diagnosis of SMBT. Conclusion: In our experience, the tumor cells of SMBT in ascites mimicked reactive mesothelial cells. The nuclear atypia of SMBTs was intermediate between that of reactive mesothelial cells and serous carcinoma. The immunohistochemistry and mucin staining using cell block were very helpful for facilitating the cytodiagnosis of SMBT.