OBJECTIVES: Most transitional cell carcinomas (TCCs) of the urinary bladder follow a relatively benign course, but some cases recur or progress rapidly. Pathological staging and grading do not necessarily have sufficient prognostic value. Herein, we assess the feasibility of using the Ki-67 index as a new prognostic indicator. MATERIALS AND METHODS: In total, 79 primary, superficial, low-grade TCCs treated by transurethral resection of the bladder tumor (TURBT) were retrospectively reviewed. Twenty-five patients(31.6%) experienced tumor recurrence. Seven of them progressed to stage T2-3 disease. Representative blocks were recut and stained with anti-Ki-67 antibody. The Ki-67 labeling index was defined as the percentage of positively labeled cells. The correlation between the Ki-67 index and tumor grade was assessed by Pearson’s Chi-square test. The prognostic significance of the ki-67 index was evaluated using the log rank test. RESULTS: In the present study, the Ki-67 index correlated well to tumor grade (p=0.0009). Recurrent tumors did not have a higher Ki-67 index (primary tumor, 13.4% vs. recurrent tumor, 9.8%, p=0.285). At a cutoff value of 10.9%(median Ki-67 index), the Ki-67 index may be a useful prognostic indicator for primary superficial low-grade TCCs. (p=0.0478). CONCLUSIONS: The Ki-67 index showed a significant difference in the log-rank test but otherwise no significant difference in the Cox proportional hazards model. Therefore further studies should be done to ensure that the Ki-67 index is the single most-important factor influencing the recurrence rate in patients with primary superficial, low-grade TCCs of the urinary bladder.