OBJECTIVE The differentiation of prostatic fibroblasts into smooth muscle cells is regarded as the key step in the development of periurethral stromal nodules. Intraprostatic stromal estrogen and transforming growth factor-beta1 (TGF-beta1) are considered to be involved in this process. We investigated whether estrogen enhances the stromal cell growth and induction of smooth muscle phenotype, and whether this process is mediated by TGF-beta1. METHODS Prostate specimens obtained from patients undergoing transurethral resection of the prostate were used for primary cell culture. Growth of the prostatic stromal cells was assessed with MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) test and cell numbers were determined by hemocytometry. The effect of estradiol on the production of TGF-beta1 protein and expression of smooth muscle markers such as smooth muscle alpha-actin (SMA) and desmin were evaluated by Western blot and immunohistochemical staining. The mRNA levels of TGF-beta1 and its receptors were analyzed by reverse transcriptase-polymerase chain reaction. We also investigated whether the enhanced expression of SMA by estradiol was mediated through the TGF-beta1 pathway using TGF-beta1 blocking antibody. RESULTS Estradiol promoted the proliferation of prostatic stromal cells by 10% to 20%. Estradiol and TGF-beta1 enhanced SMA expression. Although the levels of mRNA expression of TGF-beta1 or its receptors did not change after estradiol treatment, increased production of TGF-beta1 protein was noted. Enhanced expression of SMA by estradiol was blocked by TGF-beta1 blocking antibody. CONCLUSIONS These results suggest that estrogen stimulates the growth of prostatic stromal cells and increases smooth muscle cell markers, which may be achieved through a pathway involving TGF-beta1.