We present the case of an unexpected response by the cerebral vasculature to bypass surgery. A 66-year-old man sustained a subarachnoid haemorrhage and cerebral angiography showed a large fusiform anterior communicating artery (ACoA) aneurysm and an anomalous anterior circulation. The right A1 segment was hypoplastic and blood supply to the entire right A2 was from the left anterior circulation via the ACoA. The aneurysm was therefore not amenable to endovascular coiling or surgical clipping alone. An extracranial-intracranial bypass was performed to revascularize the territory of the right A2 independently of the ACoA to allow the latter vessel to be clipped. Although the bypass graft was patent on post-operative cerebral angiography, it was in fact only perfusing a limited, peripheral anterior cerebral artery territory. The bulk of the right anterior circulation was derived retrogradely by latent collaterals from the internal carotid and ophthalmic arteries via small dural vessels which were not apparent prior to surgery.