Patent Blue Violet dye is commonly used for selective localization of the lymphatic system before sentinel lymph node biopsy. This procedure is routinely recommended for staging the axilla in early breast cancer. However, it has long been recognized that such dye is associated with anaphylaxis. Here, we present a case of an intraoperative grade 2 anaphylaxis which commenced 20 minutes following the subcutaneous periareolar injection of patent blue for sentinel lymph node detection during segmental mastectomy. The timing of the reaction, clinical signs, and results of skin testing confirm patent blue to be the triggering agent.