A 44-year-old woman presented for sentinel node biopsy and segmental mastectomy. After anesthetic induction, a laryngeal mask airway was placed, and ventilation was satisfactory. Three minutes after isosulfan blue dye injection, ventilation became difficult. The laryngeal mask airway was removed, and an endotracheal tube was easily placed. Twenty minutes later, she became hypotensive and unresponsive to phenylephrine, ephedrine, and vasopressin. With erythema and swelling in her arm and chest, low-dose epinephrine was titrated until her mean arterial blood pressure stabilized. Serum tryptase was increased to 27.2 μg/L (normal 0.4–10.9 μg/L).