ABSTRACT Purpose: Epidemiological studies reported conflicting results about preoperative hydronephrosis in upper tract urothelial carcinoma (UTUC). This study aimed to investigate the association between preoperative hydronephrosis and pathologic features and oncologic outcomes in patients with UTUC treated by radical nephroureterectomy (RNU). Materials and Methods: This was a retrospective, single-center cohort study of 377 patients treated by RNU without perioperative chemotherapy between January 2001 and December 2014. Logistic regression, Cox regression, and survival analyses were performed. Results: Among the 226 patients with high-grade UTUC, 132 (58%) had preoperative hydronephrosis. Multivariable logistic regression revealed that hydronephrosis was independently associated with advanced pT stage (P=0.017) and lymph node or lymphovascular invasion (P=0.002). Median follow-up was 36 months (interquartile range: 20-48 months). The 3- and 5-year overall survival (OS) rates in patients with hydronephrosis were significantly lower than in those without hydronephrosis (both P <0.001). The 3- and 5-year cancer-specific survival (CSS) rates in patients with hydronephrosis were significantly lower than in those without hydronephrosis (both P=0.001). Hydronephrosis was independently associated with OS and CSS (P=0.001 and P=0.004, respectively). Among the 151 patients with low-grade UTUC, hydronephrosis was not associated with pathologic features and postoperative survival. Conclusions: Preoperative hydronephrosis was significantly associated with adverse pathologic features and postoperative survival in patients with high-grade UTUC.