Complications after intravesicalMycobacterium bovisBCG instillation Intravesical instillation of Mycobacterium bovis BCG is frequently used for non-muscle invasive high-grade urothelial cell carcinoma of the bladder. M. bovisBCG is a mycobacterium and was one of the first cancer immunotherapies. It activates the immune system and so reduces disease recurrence. Although rare, this treatment can lead to mycobacterial infections or systemic immune reactions. Diagnosing these complications is often challenging because the symptoms are non-specific. In addition, M. bovisBCG infections are hard to detect with current diagnostics, i.e. bacterial cultures, PCR and radiological imaging. Thus, proper treatment is often delayed as M. bovis does not respond to conventional broad-spectrum antibiotics. In this article we present three patients with BCG pneumonia, BCG arthritis and BCG cystitis, respectively. Subsequently, we describe the history of M. bovis BCG, its pathophysiology, the clinical presentation of its complications and diagnostics and treatment.