Compared to intravesical chemotherapy, instillations with bacillus Calmette-Guérin (BCG) may provoke more frequently local and systemic reactions. As well as irritative bladder symptoms, BCG therapy may cause systemic sideeffects,such as mild malaise, fever and, less commonly, life-threatening sepsis. It is important that risk factors for these adverse events are recognized, and that traumatic catheterization and instillations in inflamed or damaged bladders are avoided. Once recognized, virtually all side-effects can be treated successfully. Treatment options may include delaying or withholding BCG instillations,or using antipyretics and analgesics for reducing bladder symptoms. Treatment with tuberculostatic drugs for up to 6 months may be necessary in cases of severe systemic or local toxicity. In vitro, BCG is very susceptible to most antibiotics and tuberculostatic agents. Clinical trials are currently investigating whether the prophylactic use of tuberculostatic agents, such as isoniazid,or dose reduction in BCG can prevent adverse effects without impairing the antitumour efficacy of BCG.