Mucinous borderline tumors (MBTs) of the ovary are large multicystic tumors with a smooth external surface. They are composed of variably-sized cysts filled with viscous or gelatinous mucin. The inner walls are generally smooth but may show ulceration, papillary/polypoid excrescences or solid foci. Grossly, the MBTs cannot be distinguished from cystadenomas or cystadenocarcinomas. Thorough sampling is must so as not to miss the small foci of occult carcinoma (one section/cm of greatest dimension for tumors < 10 cm in size and 2 sections/cm if tumor is > 10 cm or shows intraepithelial carcinoma or microinvasive carcinoma). Sections should be taken from solid as well as any grossly unusual areas. The image shows a mucinous borderline tumor of the ovary. The tumor measured 20 cm x 16 cm x 9 cm and weighed 1260 grams. The contralateral ovary and appendix were grossly normal. Image courtesy of: Ed Uthman, MD, Houston, Texas.