Abstract Objective. This study aimed to determine the long-term risk of cancer progression of carcinoma in situ (CIS) of the urinary bladder, and whether intravesical bacille Calmette-Guérin (BCG) immunotherapy can reduce the risk of progression of CIS.Material and methods. From a prospectively enrolled cohort of bladder cancer patients treated at Århus University Hospital Skejby, Denmark, between 1994 and 2008, all 163 cases with CIS in the bladder, and a history free of invasive bladder cancer (stage T1-4) at least 1 year prior to inclusion were included in the study. Results. Median follow-up was 51 (0-253) months for progression. Initial treatment consisted of transurethral resection (TUR) alone (109 patients) or TUR plus BCG (54 patients). Twenty-eight patients underwent delayed treatment with BCG. Twenty-one patients in the TUR-alone group (19%) and 42 BCG-treated patients (51%) were free of disease at the end of follow-up (p