Uterine fibroids (UFs) are clonal, hormonally regulated, benign smooth-muscle myometrial tumors that severely affect female reproductive health, although their unknown etiology limits effective care. UFs occur fourfold more commonly in African American women than in Caucasian women, and African American women generally have earlier disease onset and greater UF tumor burden, although the mechanism of this ethnic disparity has not been identified. Recent findings have linked cancer (ie, tumor) risk to increased tissue-specific stem cell division and self-renewal and suggest that somatic mutations in myometrial stem cells (MyoSCs) convert them into tumor-initiating cells, leading to UF. Specifically, preliminary results in paraffin-embedded myometrial tissues have shown increased STRO-1⁺/CD44⁺ MyoSCs in African American versus Caucasian women. Using specific methods of flow cytometry and automated quantitative pathology imaging, a large cohort of myometrial samples were investigated to determine how the STRO-1⁺/CD44⁺ MyoSCs change with regard to a patient's race, age, parity, fibroid and hormone statuses, and the location of UFs within the uterus. We confirmed that the STRO-1⁺/CD44⁺ MyoSC population is expanded in African American women, is correlated with parity and fibroid number, and fluctuates with cyclic menstrual cycle hormone changes and age. Our data suggest that an expanded MyoSC population increases the formation of tumor-initiating cells, ultimately contributing to increased UF prevalence and burden in African American women.