The use of prophylactic agents after primary resection can decrease the incidence of tumor recurrence in patients with stage TaT1 bladder cancer. However, the long-term impact on progression to muscle invasive disease as well as on duration of survival is unknown. A combined analysis of individual patient data from previously performed European Organization for Research and Treatment of Cancer (EORTC) and Medical Research Council (MRC) randomized clinical trials was done in an attempt to answer these crucial questions. We compared immediate versus no adjuvant prophylactic treatment after transurethral resection with respect to disease-free interval, time to progression to muscle invasive disease, time to appearance of distant metastases, duration of survival and progression-free survival.