Leiomyomas, which are the commonest pelvic tumors in women, are originated from myometrial cells. Although the exact initial pathophysiologic event of the leiomyoma is not known, recent evidences suggested that the effects of sex steroid hormones in the process of tumor growth are mediated by local production of growth factors including epidermal growth factor(EGF), in- sulin-like growth factor-I(IGF-I) and insulin-like growth factor-II(IGF-II). Recently, it was sugge- sted that transforming growth factor-β(TGF-β) may play a role for growing of leiomyomas. It was known that TGF-β increase the formation of extracellular matrix. And we examined TGF-β1 expression and tried to know the effect of TGF-β1 on the extracellular matrix, collagen I and III mRNA levels in cultured leiomyoma and myometrial cells. We found that immunoreactive TGF-β1 was expressed in myometriums and in leiomyomas. There was no difference at the intensity of immunostaining between two tissues. In northern blotting, there was no difference of TGF-β1 expression at the transcriptional level in both tissues. TGF-β1 increased the mRNA levels of collagen I in cultured myometrial and leiomyoma cells at the concentration of 1 ng/ml(p<0.05), 10 ng/ml(p<0.001). And TGF-β1 increased the mRNA levels of collagen III in cultured myo- metrial and leiomyoma cells at the concentration of 1 ng/ml (p<0.05), 10 ng/ml(p<0.05). Therefore, expressed TGF-β1 may stimulate the incorporation of collagen I and III into the extracellular matrix in myometriums and leiomyomas.