Cellular mesenchymal tumors of the uterus may be divided in 2 main groups, smooth muscle and endometrial stromal. Among the former, highly cellular leiomyoma is the classic example. This tumor is not infrequently confused with an endometrial stromal tumor due to its often yellow color and soft consistency, dense cellularity, striking vascularity, not uncommon irregular margin and even rarely association with seedling cellular leiomyomas, both mimicking the infiltration of a low-grade endometrial stromal sarcoma. Cellular intravenous leiomyomatosis can also mimic endometrial stromal sarcoma due to their shared intravascular growth. A variety of histologic features typical of cellular smooth muscle including clefts and differing vasculature help in this distinction. Although endometrial stromal tumors are typically highly cellular, recent studies have expanded their spectrum to include those that are less so due to smooth muscle metaplasia, fibrous and myxoid change, and even oxyphilic cytoplasm. A subset now designated high-grade endometrial stromal sarcoma showing a t(10;17) has been characterized to show small epithelioid cells associated with brisk miotic activity, typically being CD10, ER and PR negative, and cyclin D1 positive. These tumors are juxtaposed to areas of fibromyxoid endometrial stromal neoplasia in 50% of cases. An enigmatic category of uterine mesenchymal neoplasms, often densely cellular, are those descriptively referred to as “uterine tumors resembling ovarian sex cord tumors.” Their spectrum is briefly noted as is their crucial distinction from stromal sarcoma with sex cord-like differentiation. Other tumors that rarely occur in the uterus that are densely cellular include but are not limited to undifferentiated uterine sarcoma, embryonal rhabdomyosarcoma, primitive neuroectodermal tumor, lymphoma or small cell, or undifferentiated carcinoma. In this essay, I review the most helpful morphologic, immunohistochemical, and/or cytogenetic features in the diagnosis of each one of these entities.