A 33-year-old woman had a metastasizing leiomyoma in which mitotic figures could not be found. The tumor was composed of well differentiated smooth muscle indistinguishable from uterine leiomyoma or normal myometrium by light and electron microscopy. The S-phase fraction of the tumor, measured by in vitro tritiated thymidine labeling, was near the upper limit of the range seen in seven ordinary uterine leiomyomas and was distinctly lower than that in a uterine leiomyosarcoma. The content of estrogen receptors in the cytosol was within the range seen in uterine leiomyomas. Metastasizing leiomyoma is an actively proliferating neoplasm of mature smooth muscle that appears to be hormonally responsive. The tumor in our patient showed no clear differences from ordinary leiomyomas in findings relating to the rate of cellular proliferation, morphology, or estrogen receptor content.