OBJECTIVE To report the long-term results of bacille Calmette-Guérin (BCG) intravesical therapy in relation to disease progression and recurrence in primary T1 high-grade (HG) bladder cancer (BC) confirmed by central pathological review. PATIENTS AND METHODS In all, 136 patients from two university centres (Rotterdam, n = 49; Toronto, n = 87) were diagnosed with primary T1HG BC. One experienced uro-pathologist reviewed all slides, ensuring all cases were indeed HG and that muscle was present in all specimens. Patients were treated with BCG induction (six instillations) after transurethral resection (TUR) of the tumour and followed with cystoscopy and urinary cytology. Predictors for recurrence, progression and survival were assessed with multivariable Cox regression models. RESULTS Mean (range) follow-up was 6.5 (0.3-21.6) years. There were no significant differences for recurrence (P = 0.52), progression (P = 0.35) and disease-specific survival (DSS) (P = 0.69) between the two centres. Among the cohort, 47 patients (35%) recurred and 42 (30.9%) progressed with a median time to progression of 2.1 years; 16 (38%) of these progressions occurred ≥ 3 years after the initial BCG course; 22 (16%) patients who progressed died from BC. Overall, 96 (71%) patients had no evidence of disease at the last follow-up. Carcinoma in situ was the only independent predictor for recurrence in multivariate analysis (P = 0.011). No independent predictors were found for progression. CONCLUSIONS Conservative treatment with BCG is a valid option in primary T1HG BC. Nevertheless, the aggressive nature of T1HG BC is evident in the fact that 30% progressed, with a high proportion of these progression events occurring ≥ 3 years after BCG. Caution should be exercised when relying on the long-term effects of BCG, and close follow-up of these patients should not be neglected.