Background: Disease progression after Bacillus Calmette‐Guérin (BCG) instillation therapy for bladder cancer is not rare. The purpose of this study was to evaluate the outcome of patients treated with BCG for superficial bladder cancer, focusing on the patients who developed invasive disease during follow‐up. The possible mechanism and risk factors for early progression after BCG therapy are discussed.Methods: A total of 25 patients with superficial bladder cancer (pTa, pT1, and/or pTis) were treated with intravesical BCG instillation (80 mg in 80 mL saline) once a week for eight weeks. Four of the 25 patients received maintenance therapy with BCG (once a month for 3 to 10 months). Patients were followed every three months and underwent cystoscopy, biopsy, and urinary cytology at these intervals. Disease progression was defined as invasion to muscle or prostate, or development of metastatic disease. Clinicopathological features of the patients, especially those with progression, were analyzed.Results: Progression was observed in six of the 25 patients (including four of 19 patients with carcinoma in situ and two of five patients treated prophylactically with BCG). The average time to progression was 8.7 months. Four patients died of cancer despite intensive treatment. Two patients are alive: one without evidence of disease after cystectomy and the other with metastatic disease.Conclusions: Proper patient selection, careful follow‐up, and immediate aggressive therapy in case of progression were considered to be important factors to obtain satisfactory results with BCG therapy for bladder