Uterine leiomyomas are the most common gynecological benign tumor and greatly affect reproductive health and wellbeing, but the pathophysiology and epidemiology of uterine leiomyoma are poorly understood. One of the major reasons for the slow progress in leiomyoma research is the lack of a good in vivo model system. We therefore aimed to develop a novel model by transplanting human uterine leiomyoma xenografts in an immunodeficient mouse strain (NOD/SCID/γc-null: NOG). Human uterine leiomyoma tissues were cut into small pieces and inserted subcutaneously into the right and left flanks of NOG mice. Estrogen supplementation was needed to maintain the features of uterine leiomyoma in xenografted tissues. After 4 weeks or 8 weeks of transplantation, xenografted tissues were harvested and analyzed regarding tissue morphology, collagen content, and proliferation and apoptosis of uterine leiomyoma smooth muscle cells. The xenografts that were harvested after 4 weeks and 8 weeks retained the histological architecture of original uterine leiomyoma tissue both in cellular and collagen components. The expression profiles of key markers of uterine leiomyoma were also maintained, including estrogen receptor, progesterone receptor, and α-smooth muscle actin, as judged by immunohistochemical staining. The proportion of proliferating cells was significantly increased (1.5-fold) in the xenografts after 8 weeks of transplantation, whereas that of the apoptotic cells remained unchanged. Importantly, the reproducible results were obtained with the tumor tissues derived from six patients. The present in vivo model may provide a useful tool for development of novel therapeutic strategies for uterine leiomyoma.