The term STUMP (smooth muscle tumor of uncertain malignant potential) is used to describe a uterine smooth muscle tumor in which the histologic features leave some uncertainty as to the tumor's biologic potential. Histologic features that might prompt putting a tumor into this category include a high mitotic rate, atypical mitotic figures, tumor cell necrosis, and nuclear atypia. Although each of these is typically evaluated separately, they must be assessed as a group to establish the most appropriate diagnosis. The term STUMP is only used as a diagnostic category when the findings are equivocal and it is not possible to place a tumor into one of the defined diagnostic categories. Examples where this might be necessary include tumors with a very high mitotic rate (>20 mitotic figures/10 high-power field), tumors where the type of necrosis is questionable, or tumors with significant nuclear atypia where there are atypical mitotic figures or the mitotic rate is elevated but not to the level held to be diagnostic of leiomyosarcoma. In some cases, diffuse strong staining for p16 can support placing a smooth muscle into the STUMP category. Most tumors categorized as STUMPs have a benign clinical evolution, but occasional STUMPs recur or metastasize.