A 72 year old male Caucasian patient was referred to our unit with a metastatic bony deposit involving his left proximal femur from a known primary prostatic carcinoma. Commonly, prostatic metastases are sclerotic in nature. However detailed radiological evaluation demonstrated a large lytic lesion in this case. Due to the high risk of an impending pathological fracture, the decision was taken to perform prophylactic intramedullary nailing of the left femur. In order to reduce the increased intramedullary pressure during nail insertion, venting of the distal end of the femur was performed. The patient did not have any other significant past medical history and he was not known to be allergic to any medication. He had surgical procedures for other conditions in the past requiring general anaesthesia and had an uneventful recovery in every case. During the procedure, the patient suffered a cardiac arrest. Fat embolism was initially thought to be the cause of this event due to the increased intramedullary pressure during reaming and nail insertion. Following successful resuscitation, a marked generalized erythematous rash was noticed which lasted for about thirty minutes. This raised the concern of a possible hypersensitivity reaction to one of the anaesthetic agents. Our patient received in total eight different drugs namely propofol, atracurium, morphine, ondansentron, dexamethasone, gelofusine, paracetamol and diclofenac. Interestingly, it was noted that the cardiac arrest occurred following a dose of atracurium and an infusion of gelofusine. Consequently, it was thought that either one of those two agents could have been responsible for the anaphylactic response. Serum mast cell tryptase levels taken one hour following the event, demonstrated a level of 190 ng/ml, which dropped to 60 ng/ml upon repeating the levels twelve hours later (normal range 3–23 ng/ml). The raised levels of mast cell tryptase were consistent with mast cell degranulation. The latter strongly suggested a hypersensitivity reaction. Furthermore, allergy skin prick tests were performed to all the drugs used during the procedure. Interestingly, they showed a positive reaction to gelofusine only, confirming it as the causative agent.