We present the case of a 59‑year‑old Taiwanese female who is married and retired, and was diagnosed with invasive ductal carcinoma in her right breast at stage cT1cN0 (Stage IA). She visited us for further treatment and underwent breast‑conserving surgery after communication. During the surgery in the operating room, Patent Blue V was injected to localize the sentinel lymph nodes. At the same time, the patient developed blue‑colored urticaria around her whole body skin after the blue dye injection. This rare reaction alerted us to pay attention to the patient’s vital signs and prevent a severe allergic reaction, but her vital signs remained stable throughout the operation process. Although her vital signs were stable when she returned to the nursing ward, the patient required more time to recover, possibly due to her allergic predisposition.