Uterine neoplasms composed of an admixture of smooth muscle and adipose tissue are rare and have been designated as lipoleiomyomas. The origin of this tumor is stil controversial and it has not been sufficiently studied. The aim of our study was to investigate the immunohistochemical phenotype of fat cells in uterine lipoleiomyomas so as to clarify their origin. Archived tissue samples of 10 uterine lipoleiomyomas were selected and analyzed immunohistochemically for vimentin, desmin, and HMB-45 expression. The patients ranged from 31 to 63 years of age (mean age 53.5±9.9). Seven tumors which affected the uterine corpus, showed intramural location; while two cases were subserosal, and one was in the cervix. All tumors were constituted by irregular bundles of smooth cells and mature large adipose cells. The amount of adipose component varied from 5 to 95% of the tumor mass. Cytological atypia and necrosis were not seen. Immunohistochemical investigations revealed obvious reactivity to vimentin and desmin in perivascular immature mesencyhmal cells and tumoral smooth muscle cells. Adipose cells in the tumors demonstrated uniform vimentin expression and inconsistent desmin immunoreactivity. All adipose cells were negative for HMB-45 antigen. However, HMB-45 antigen was weakly positive in spindle shaped tumor cells of two cases. In our study, the immunohistochemical findings suggest a complex histogenesis for these tumors, which may arise from perivascular immature mesencyhmal cells or direct transformation of smooth muscle cells into adipocytes by means of progressive intracellular storage of lipids.