BACKGROUND: Increased risk of intracranial aneurysm rupture with subsequent subarachnoid haemorrhage (SAH) is related to several possible factors, including first-grade familial aneurysms. METHODS AND RESULTS: Here we present a case of one ruptured and one unruptured identical twin-like anterior communicating artery (ACom) aneurysm present in two male siblings, without any proven genetic predisposition. According to evidence-based scores, aneurysm rupture risk for the younger (51 years old) sibling (with an unruptured ACom aneurysm) would be low, leaving the decision for either treatment or clinical follow-up to the discretion of the treating physician. CONCLUSIONS: From a clinical neurosurgical perspective, however, especially since his older brother suffered an aneurysmal SAH at age 57 (i.e. 6 life-years later) from a twin-like-appearing ACom aneurysm, in our opinion rupture seems almost inevitable, thus prompting treatment at an early stage.