Purpose: This study evaluated the rate and predictors of bacteriuria in patients who underwent transurethral resection of bladder tumor (TURBT) and Bacillus Calmette-Guerin (BCG) treatment.Materials and Methods: We prospectively evaluated 266 patients who underwent TURBT and intravesical BCG treatment between January 2017 and September 2019. Urinalysis and urine culture were performed at the baseline, one to two weeks after TURBT and weekly during the BCG treatment period. The primary outcomes were the bacteriuria rates in BCG-treated patients, while the secondary outcomes were the risk factors of bacteriuria during BCG treatments. Results: Of the 266 patients, the rate of bacteriuria was 4.5% before TURBT, 5.3% in the postoperative period, and 24.4% in the BCG treatment period. After BCG instillation, urinalysis showed that 204 and 38 patients tested positive for pyuria and nitrite, respectively. Multivariate analysis indicated that the risk factors associated with bacteriuria during BCG treatment included age (odds ratio [OR]: 1.06; p=0.003), sex (female) (OR, 5.41; p=0.007), diabetes mellitus (DM) (OR, 2.82; p=0.023), postoperative bacteriuria (OR, 8.08; p=0.032), bacterial counts>100/?l in urine flow cytometry (OR, 29.72; p<0.001), and positive urine nitrite test (OR, 6...