A potentially dangerous complication occurring during deep regional hyperthermia is described. A patient receiving epidural analgesics for pain caused by a large pelvic recurrent rectal tumour was treated by hyperthermia induced by electromagnetic radiation. The epidural infusion pump failed during heating and further analgesics were administered by bolus injections into the epidural space. Following the second bolus injection, a severe drop in arterial blood pressure was observed. The most likely multifactorial pathogenesis is discussed and measures to avoid such an event are recommended.