Twenty patients with congenital cerebral aneurysms, studied on a 1.5 tesla MR unit, were also evaluated by angiography and/or computed tomography (CT) in order to establish the relative sensitivity of magnetic resonance imaging (MRI) in the demonstration of such aneurysms. Of the 27 aneurysms found on MRI, all but 4 were seen without the aid of the angiographic study. The four seen in retrospect included 3 out of 4 in the smallest size category (less than 5 x 5 mm) and one in next larger (5 x 5 mm to 10 x 10 mm). When more than one aneurysm was present or when an aneurysm and an arteriovenous malformation was present, MRI proved to be useful in demonstrating which bled. The presence of both subacute and acute clot identified the lesions that had bled.