Intravesical therapy with bacillus Calmette-Guerin (BCG) has proved to be more effective in the prophylaxis and treatment of superficial bladder tumors and carcinoma in situ than most chemotherapeutic agents. Compared to intravesical chemotherapy, instillations with BCG provoke more local and systemic reactions. In addition to the commonly induced granulomatous inflammatory changes in the bladder, which produce irritative symptoms, this therapy may cause systemic side effects varying from mild malaise and fever to, in rare instances, life-threatening or fatal sepsis. We report the incidence and varieties of toxicities in 2,602 patients treated with intravesical BCG. Side effects are classified according to local and systemic toxicity. Treatment options vary according to the severity of toxicity from delaying or withholding instillations to treatment with antituberculous drugs for up to 6 months. In general, 95% of the patients have no serious side effects. Recognition of risk factors, particularly traumatic catheterization or concurrent cystitis, that result in systemic BCG absorption, as well as the prompt and appropriate treatment of early side effects should significantly decrease the incidence of severe toxicity.