A 62 year old female undergoing sentinel lymph node biopsy and right lumpectomy was administered methylene blue alongside anaesthetics. Two minutes after administration of methylene blue, the patient entered anaphylactic shock with hypotension, discolouration, and angioedema. Fluid, steroids, and vasopressors were administered and the patient stabilized, at which point the operation continued. Serum taken an hour after anaphylaxis demonstrated a tryptase of 34 ng/ml. A sample six hours later demonstrated prolonged elevation of tryptase at 58 ng/ml. A baseline sample was within normal limits (<12 ng/ml) at 3 ng/ml. Skin prick and intradermal testing was negative for penicillin, ancef, latex, rocuronium, and propofol. Intradermal testing, however, was positive for methylene blue at 1/10, although a non-irritating concentration has yet to be established.