Anisocoria during general anaesthesia is rare. The combination of anisocoria and absence of any reaction to light is highly suspicious of an intracerebral lesion. This article reports the case of a patient with known Adie's syndrome where anisocoria led to an unnecessary immediate interruption of anesthesia. A preoperative examination, including inspection of the pupils, frequent perioperative check ups and documentation of diagnostic findings, even if negative, has to be standard for every anesthesia.