Objectives: To examine the usefulness of smoothelin - a new immunohistochemical (IHC) marker that is expressed predominantly in visceral smooth muscle - in recognizing muscularis propria (MP) in transurethral resection (TUR) and matched cystectomy specimens and to compare the pattern of its expression in muscularis mucosae (MM) and MP in radical cystectomy specimens. Methods: IHC staining for smoothelin was performed in 49 cases of urothelial carcinoma removed by radical cystectomy (16 had undergone TUR before the cystectomy). Results: In cystectomy specimens, smoothelin staining in the MP was strong (+3), moderate (+2) and weak (+1) in 49%, 44.9% and 6.1% of cases, respectively, whereas smoothelin positivity in the MM was absent and weak in 77.6% and 22.4% of cases, respectively. In TUR specimens, smoothelin immunoreactivity was moderate to strong in 68.8% and weak in 6.3% of cases and all of them proved to have MP invasion in cystectomy specimens. Conclusion: Smoothelin is a useful marker for the detection of MP in TUR specimens. Moderate to strong smoothelin staining of the muscles included in TUR specimens and split by the tumor is a sign of MP invasion. It may be useful in cancer staging and treatment decision making