Recent studies have suggested that a deficiency of alpha 1-antitrypsin may be a genetic risk factor for the development of intracranial aneurysms and arterial fibromuscular dysplasia. The authors report a 16-year-old girl with a history of lung disease who suffered a cerebral hemorrhage due to the rupture of a giant intracranial aneurysm arising from the middle cerebral artery. This fusiform aneurysm was associated with fibromuscular dysplasia of the intimal type. She was found to have an unusual alpha 1-antitrypsin deficiency (PiMP phenotype). This case provides further evidence of an underlying arteriopathy in alpha 1-antitrypsin deficiency.