Abstract Objective The primary aim was to search for lower doses of Bacillus Calmette-Guerin (BCG) that are effective and have lower toxicity. Methods A low dose of BCG 27 mg was compared with BCG 13.5 mg, using mitomycin C (MMC) 30 mg as the third arm of comparison. A total of 430 patients with intermediate-risk superficial bladder cancer were randomised into three groups. Instillations were repeated once a week for 6 wk followed by another six instillations given once every 2 wk during 12 wk. Results There was a significantly longer disease-free interval for BCG 27 mg versus MMC 30 mg ( p = 0.006). There were no statistically significant differences between BCG 27 mg and BCG 13.5 mg ( p = 0.165) or between BCG 13.5 mg and MMC 30 mg ( p = 0.183). Cox proportional hazards regression showed that disease-free interval in the multivariate analysis was significantly better for primary disease and treatment with BCG 27 mg. There were no significant differences among the three groups with regards to time to progression and cancer-specific survival time. Local and systemic toxicity were higher in both BCG treatment groups. Conclusions One third of the standard dose, BCG 27 mg, seems to be the minimum effective dose as adjuvant treatment for intermediate-risk superficial bladder cancer, being more effective than MMC 30 mg. One sixth of the standard dose, BCG 13.5 mg, has the same efficacy as MMC 30 mg but it is more toxic.