OBJECTIVES: Hydronephrosis in patients with bladder cancer is caused by tumour at the ureteral orifice, secondary ureteral tumours, intramural or extravesical tumour infiltration, or compression of the ureter. This study investigated the prognostic impact of hydronephrosis in bladder cancer. METHODS: A series of 788 patients were treated with radical cystectomy with curative intent for transitional cell carcinoma of the bladder without neoadjuvant/adjuvant radiotherapy/chemotherapy between January 1986 and September 2003. All patients had a complete follow-up until death or until the study's end date. Survival rates were calculated using the Kaplan-Meier method. A multivariate analysis with a Cox regression model was performed with respect to potential influencing factors. RESULTS: A total of 108 patients (13.7%) had unilateral and 25 patients (3.2%) had bilateral hydronephrosis. The rate of organ-confined tumours was significantly higher in patients without hydronephrosis (67.9% vs. 37.6%; p<0.001). Forty-three (32.3%) of the 133 hydronephrotic patients had a tumour involving the ureteral orifice. In this group the rate of organ-confined tumours was significantly higher than in the other patients with hydronephrosis (53.5% vs. 30.0%; p=0.009). In the multivariate analysis, preoperative hydronephrosis was determined as an independent prognostic marker for recurrence-free survival besides the pT classification and lymph node status (p=0.0015). The etiology of hydronephrosis did not affect the tumour-specific survival. CONCLUSIONS: Hydronephrosis at the time of diagnosis of bladder cancer is associated with a high probability of advanced tumours. It is an independent prognostic factor for recurrence-free survival.