Bacillus Calmette-Guérin (BCG) therapy using the Southwest Oncology Group (SWOG) maintenance protocol is the standard in non-muscle-invasive bladder cancer (NMIBC). Maintenance with monthly instillations is also widely used, but evidence comparing the two maintenance protocols is scarce.To compare monthly and SWOG instillation schedules in maintenance BCG therapy.We retrospectively identified patients with NMIBC treated with maintenance BCG according to either the monthly or the SWOG instillation regimen in two tertiary care centers in Finland between 2009 and 2019.We compared discontinuation rates of the monthly and SWOG maintenance protocols due to toxicity, and recurrence and progression rates by protocols. Baseline characteristics were compared with the Wilcoxon rank sum test, chi-square test, and Fisher's exact test. The Kaplan-Meier method and Cox proportional hazards model were used to evaluate the discontinuation of BCG due to toxicity and oncological efficacy.We identified 723 patients, of whom 545 (75%) and 178 (25%) received maintenance according to the monthly and SWOG protocols, respectively. The median follow-up time was 66 (interquartile range: 45-99) mo. In the monthly and SWOG groups, 131 (24%) and 50 (28%) patients, respectively, discontinued BCG due to toxicity, with no difference in a univariate or multivariate analysis (hazard ratio 1.01, 95% confidence interval [CI]: 0.73-1.40, p = 0.940). The 5-yr recurrence-free survival rates in the monthly and SWOG groups were 65% (95% CI: 61-69%) and 71% (95% CI: 64-79%, p = 0.370), respectively. The 5-yr progression-free survival rates were 89% (95% CI: 86-92%) and 91% (95% CI: 86-96%, p = 0.240), respectively.Monthly maintenance is a comparable alternative to the SWOG protocol.In this study, we compared two schedules of intravesical bacillus Calmette-Guérin (BCG) treatment used in the treatment of non-muscle-invasive bladder cancer. We found that there were no significant differences between the two instillation schedules in terms of tolerability or efficacy.