We report a rare but notable case of cerebral air embolism complicating transthoracic intrapulmonary injection of an imaging agent used to locate sentinel lymph nodes. After a bolus injection of 2 mL of iopamidol into the peritumoral area with a 23-gauge needle, the patient complained of complete paralysis on his left side. Intraaortic gas was detected by computed tomography immediately after the injection. The patient recovered spontaneously without any additional complication. Surgeons should be aware of this rare but possible complication during sentinel lymph node assessment.