A ruptured cavernous carotid aneurysm (CCA) with carotid cavernous fistula can appear as a benign headache but progress to a swollen and bloodshot eye overnight. A 66-year-old woman visited emergency department with sudden onset of pain behind her left forehead and vomiting. She was treated for a migraine-like headache and discharged. She presented again on the next day with a persistent headache and a swollen left eye with blurred vision. An ophthalmologic examination revealed erythema of the left lid and chemosis at the temporal and lower bulbar conjunctiva. A cranial nonenhanced computed tomography (CT) scan had been performed at her previous visit. The scan exhibited a nodular mass lesion involving the left cavernous sinus. CT angiography was subsequently used to determine that the lesion was a giant aneurysm in the left cavernous internal carotid artery, causing enlargement of the left ophthalmic veins. The symptoms of her left eye rapidly progressed to severe chemosis, edematous change over periocular region, and limited movements after 8 h. The patient received emergent lateral canthotomy and inferior cantholysis to avoid acute orbital compartment syndrome and was subsequently treated with stent-assisted coil embolization. A ruptured CCA is an urgent condition that requires rapid assessment of both cranial vascular and ocular lesions. A history of sudden onset headache with a nonpainful acute unilateral red eye may serve as a clue to prompt additional diagnostic studies and ophthalmologist evaluation. Adequate radiological studies and early endovascular intervention can reduce the likelihood of permanent ocular injury and vision impairment.