Intravesical bacillus Calmette-Guérin (BCG) therapy is the standard prophylaxis for recurrence of non-muscle-invasive bladder cancer (NMIBC). The aim of this study was to confirm the recurrence- and progression-preventing efficacy and safety of 12 times monthly BCG maintenance therapy for NMIBC. This study included 126 patients diagnosed with Ta, T1 and carcinoma in situ bladder cancer between January 2000 and December 2009. Thirty-four patients in the no maintenance group received a single 6-week course of intravesical immunotherapy after transurethral resection of the bladder tumor (TUR-BT). Ninety-two patients in the maintenance group received a 12-month course after a single 6-week course of intravesical immunotherapy. Recurrence, progression and side effects were assessed. End-points were recurrence-free survival (RFS), progression-free survival (PFS) and disease-specific survival (DSS). The estimated median RFS was 87 months (95% CI 53.0-120.9) in the maintenance group and 48 months (95% CI 0-96.8) in the no maintenance group (log-rank test; P=0.002). The 2-year cumulative RFS rates were 77.3% in the maintenance group and 52.6% in the no maintenance group. Median PFS and DSS were not estimable in both groups. The 2-year cumulative PFS rates were 91.1% in the maintenance group and 80.5% in the no maintenance group (log-rank test; P=0.178). The 2-year cumulative DSS rates were 97.7% in the maintenance group and 91.4% in the no maintenance group (log-rank test; P=0.111). The overall side effects were 40.2% in the maintenance group and 44.1% in the no maintenance group. Monthly maintenance immunotherapy appears to increase RFS rates in high-risk NMIBC. Further study is required to evaluate the efficacy of BCG monthly maintenence for increasing the PFS rate.