Two patients with surgically and angiographically proved partially thrombosed giant aneurysms of the middle cerebral artery were studied with computed tomography (CT) and magnetic resonance (MR) imaging. MR and histopathologic findings were correlated. The central location of methemoglobin, with its high intensity (surrounding the patent lumen, seen as signal void), in giant aneurysms is directly opposite the initial peripheral appearance of methemoglobin in extra-aneurysmal intracerebral hematomas. More peripherally, the thrombosed portion of the lumen is layered with intensities that represent stages of clot (methemoglobin and hemosiderin). Three characteristics enable differentiation of giant aneurysms from intracerebral hematoma: signal void in residual patent lumen; laminated, staged thrombus with intervening layers of hemosiderin and methemoglobin that is initially centrally, rather than only peripherally, located; and signal void in the vessel from which the aneurysm arises. Hemorrhage from prior bleeding can be readily identified and separated from perianeurysmal edema on MR images. MR appears to be a specific, noninvasive method for diagnosing partially thrombosed giant intracranial aneurysms and is superior to CT and angiography in characterizing these lesions.