Serous cystadenomas of the ovary are thin-walled unilocular cysts that are lined by ciliated pseudostratified cuboidal or columnar epithelium (resembling that lining the fallopian tube). Hobnail cells may be present. The epithelium is supported by variable amounts of spindle cell stroma. Cytologic atypia is not seen. Psammoma bodies are present in about 30% of cases. The cyst wall may show focal papillary architecture. Papillae, when present, are also lined by a single layer of cells and lack cytologic atypia, architectural complexity, or invasion.