The S100P protein stimulates cell proliferation and survival, thereby contributing to tumor progression. The purpose of this study was to evaluate S100P expression in the three subtypes of mucinous cystic tumors, cystadenomas, borderline tumors, and adenocarcinomas. The authors examined nuclear S100P expression in 60 mucinous ovarian tumor specimens, including 24 specimens of mucinous cystadenoma, 15 of borderline tumors, and 21 of adenocarcinomas. Immunohistochemistry revealed S100P expression followed one of three patterns: (1) Expressed in most nuclei of mucinous epithelial cells, (2) sporadic (spotted or patchy) expression, or (3) absent or rarely expressed in the nuclei of mucinous epithelial cells. Most adenomas showed the first expression pattern, and borderline tumors often showed a patchy expression pattern. Adenocarcinomas generally demonstrated absence of S100P expression. These data suggest that S100P is a useful histological marker to differentiate between benign, borderline, and malignant mucinous tumors of the ovary.