Objective To study the expression and significance of PIM-1 in bladder transitional cell carcinoma.Methods Expression and localization of PIM-1 in human normal and malignant bladder specimens were examined by Immunohistochemistry and the PIM-1 staining score was also compared with several clinicopathologic parameters; then PIM-1 expression was also validated in five bladder cancer cell lines by western blotting and immunohistochemistry assays.Subsequent knockdown of PIM-1 was achieved by lentivirus encoding small interfering RNA,and the effect of PIM-1 on bladder cell survival was further assessed by colony formation assays,finally the anti-apoptosis protein bcl-2 and p-BAD were examined by WB.Results When compared with normal epithelium,PIM-1 was overexpressed in bladder cancer epithelium (2/21 to 38/45) (Pvalue ＜ 0.01 ),and the expression level was even higher in invasive bladder cancer than Non-invasive bladder cancer specimens ( 19/20 to 19/25 ) (P value ＜0.01 ).PIM-1 was also detected in all the bladder cancer cell lines examined.Moreover,the knockdown of PIM-1 significantly inhibited bladder cancer cell growth which associated with the reduction of bcl-2 and p-BAD.Conclusion PIM-1 is up-regulated in bladder transitional cell carcinoma specimens and associated with the tumor stage,more importantly PIM-1 is much higher in invasive bladder transitional cell carcinoma than no-invasive bladder transitional cell carcinoma.Further more,PIM-1 is highly expressed in bladder cancer cell lines and downregulated PIM-1 can inhibit the growth of bladder cancer cells and is involve in the increase of apoptosis,therefore PIM-1 may play a role in bladder cancer initiation and progression which may be a potential candidate as target for novel therapy in bladder cancer.