Acute 3rd cranial nerve palsy (in the absence of raised intracranial pressure or trauma) is usually painless and due to small vessel ischaemia (i.e. underlying vascular disease including diabetes). It is usually pupil-sparing as parasympathetic fibres are more resistant. Traumatic or compressive causes tend to involve the pupil. If pain is a feature then acute expansion of a berry aneurysm must be excluded. It usually arises from the distal internal carotid at the level of the posterior communicator projecting posteriorly. This case is unusual as the terminal ICA at the anterior cerebral origin is the site (it is also unusual to find berry aneurysms here).