To investigate the role of a recently cloned cell proliferation-related gene, mig-2 (mitogen-inducible gene-2), in the growth of uterine leiomyomas, this gene's expression at mRNA and protein levels was examined in normal myometrium, leiomyomas, and leiomyosarcomas of the uterus. Northern blotting, Western blotting, and immunohistochemical staining demonstrated that mig-2 expression was increased in leiomyomas compared with normal myometrium, especially during the secretory phase of the menstrual cycle. In contrast, the mig-2 expression was greatly decreased in leiomyosarcomas. Direct sequencing of the whole coding region of mig-2 cDNA and Southern blotting revealed that mig-2 alterations, such as mutations, rearrangement, and amplification, were not present in either leiomyomas or leiomyosarcomas. These findings suggest that mig-2 expression is transcriptionally elevated in leiomyomas and could be involved in its hormone-mediated growth of leiomyomas of the uterus.