Immediate adjuvant Mitomycin C (MMC) instillation is routine practice in the treatment of superficial bladder cancer. Despite relative safety we describe a case of MMC extravasation after intravesical instillation. This resulted in severe continuous pain in the pelvic region without tendency of spontaneous healing, and required surgical debridement. To assess perivesical soft tissue injury prior to surgery MRI imaging turned out to be more accurate than computer tomography. Suggestions about how to avoid, diagnose and treat this symptomatic extravasation are made.