The first aneurysms explored by such pioneers of neurosurgery as Cushing and Dandy were the giant intracranial aneurysms. These giant aneurysms present many therapeutic difficulties and, because of their unique anatomical features and size, may present in a multitude of ways. With the advent of specialized imaging techniques such as computed tomography (CT), mag netic resonance imaging (MRI) and selective angiography, preoperative diag nosis today is most often accomplished without difficulty. However, com pletely thrombosed giant aneurysms may mimic other lesions with mass effect (such as basilar meningiomas, chordomas or chondromas)