Serous cystadenomas of the ovary may occasionally show foci that meet the criteria for serous borderline tumor. If such foci represent < 10% of the tumor volume, the tumor is designated serous cystadenoma with focal epithelial proliferation. This serous cystadenoma was composed of a smooth-walled unilocular cyst except for one focus (shown here) with papillary architecture. Some of the papillae had simple architecture with no secondary branching. Others showed a higher level of complexity (as shown here). Such cases are designated serous cystadenoma with focal epithelial proliferation.