BackgroundBilateral third cranial nerve palsy has only been reported in a handful of conditions including some with inflammatory, tumoural and vascular causes. An urgent imaging study is mandatory to rule out vascular aetiology, mainly aneurysmal subarachnoid haemorrhage (aSAH).Case presentationA 28-year-old Hispanic woman presented to the emergency department with a 21-day history of a sudden-onset and severe headache that on three occasions was accompanied by transient loss of awareness, the last episode occurring a week previously. The simple CT image showed minimal bleeding at the level of the perimesencephalic cisterns, with evidence of SAH. An angioCT revealed a 5×6 mm bilobed saccular aneurysm of the right posterior communicating artery and a 2×2 mm saccular aneurysm in the posterior left communicating artery.ConclusionsA mirror aneurysm is found in 2–25% of aSAH cases. To date there is no consensus about the optimal management of patients with these findings.LEARNING POINTSThe presence of third cranial nerve palsy should always raise the suspicion of an aneurysm.Subarachnoid haemorrhage is the most common cause of a thunderclap headache.Aneurysmal subarachnoid haemorrhage requires surgical management in all cases.