Objective: To detect the expression of second mitochondria-derived activator of caspase (Smac) in bladder cancer and discuss its clinical significance. Methods: Smac was detected in 15 specimens of normal bladder epithelium and 72 specimens of bladder cancer by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry at the level of gene and protein,respectively. Results: The differences of both Smac protein and mRNA expressions between normal mucous membrane of bladder and grade Ⅰ bladder cancer had no statistical significance ( P ＞ 0.05). The expressions of Smac protein and its mRNA in bladder cancer decreased gradually with the advance of bladder cancer ( P ＜ 0.01 and P ＜ 0.05, respectively ). In invasive bladder cancer, the expressions of Smac protein and its mRNA were higher than those in superficial bladder cancer (P＜0.01). Conclusions: Normal bladder epithelium has high expression of Smac while bladder cancer has low expression of Smac. The expression of Smac is closely related to the grade and stage of bladder cancer. Detection of Smac expression helps to judge the grade and stage of bladder cancer and Smac gene might become a valid target for gene therapy of bladder cancer.