Objective:To investigate the expression of KAI1 gene in the urothelial cancer tissues and its relationship with the invasion and metastasis of urothelial cancer.Methods: The expression of KAI1 mRNA was detected by realtime fluorescent quantitative(RFQPCR) in urothelial cancer tissues and normal mucosa of urinary tract. The KAI1 protein expression was detected by immunohistochemistry(IHC) method in bladder transitional cell carcinoma tissues and the paired normal mucosal tissues.Results: QRTPCR showed that the average level of KAI1 mRNA in the urothelial cancer tissues was significantly lower than that in the normal bladder tissues (P<0.01); moreover,the increase of pathological grades and clinical stages and the development of lymphatic metastasis were associated with the decrease of KAI1 expression,with significant difference found between the different groups(P<0.05 or P<0.01). The protein expression of KAI1 in the urothelial cancer tissues was significantly lower than that in the normal bladder tissues(P<0.01). The protein expression of KAI1 was decreased with the increase of pathological grades(P<0.05 or P<0.01).We also found that higher expression of KAI1 was associated with superficial invasion (P<0.05) and the presence of lymphatic metastasis (P<0.05).Conclusion: The downregulation of KAI1 gene is associated with differentiation,infiltration,and lymphatic metastasis of urothelial cancer,which might serve as an effective indicator for malignancy,metastasis and prognosis of urothelial cancer.