MR studies of 17 patients with 19 intracranial aneurysms are reviewed. All patients also underwent CT and angiography. MR has been able to visualize the aneurysms in all cases. Aneurysms present various MR appearances because of flow characteristics, thrombosis in different stages of organization, calcific and ferric deposits. Based on MR signal changes it is possible to distinguish between flow effects and histopathological components such as thrombosis. Flow patterns are complex and sometimes it is difficult to define the cause responsible for intraluminal signal. MR allows a precise definition of perilesional brain tissue and demonstrates associated lesions. Angiography remains the definitive procedure in the diagnosis of small aneurysms, but shows only that part of the lesion in continuity with the circulation. MR clearly delineates the size, the residual lumen and the extra-axial location of giant aneurysms. In completely thrombosed aneurysms, when CT suggest a tumor, MR is able to demonstrate the vascular nature of the lesion.