WITH REFERENCE TO vascular malformations, the termcavernoushas architectural as well as histologic connotations. It refers to a compact pattern of growth wherein no intervening brain parenchyma is evident, as well as to the histological nature of the vessels, which are hyaline and collagenous in appearance, lacking the microscopic features of arteries or veins. Historically,cavernous angiomahas been defined as exhibiting both features. Twenty-five patients with neurological symptoms and neuroimaging abnormalities who underwent surgery for cerebral vascular malformations between 1987 and 1990 satisfied the following study criteria: their lesions were angiographically occult and both magnetic resonance imaging (MRI) and histological sections were available for review. The patients' ages ranged from 4 to 49 years (mean, 30 years), the male to female ratio being 1:2. Two thirds of the lesions were supratentorial in location and all were intraparenchymal. All patients had clinical improvement after resection. In 24 of the 25 lesions, the vascular channels were histologically cavernous in nature; one inadequate specimen precluded classification. Three demonstrated a purely compact or cavernous pattern, 20 a mixed cavernous and racemose pattern, and one a purely racemose pattern. The authors conclude that 1) histologically cavernous lesions are the commonest form of occult vascular malformation; 2) a purely compact or cavernous architectural pattern is uncommon, most lesions showing a partially racemose architecture; 3) some histologically cavernous malformations possess a capillary component; 4) clinical growth of cavernous malformations may have its basis in intraluminal thrombosis and subsequent recanalization; 5) the T2-weighted MRI pattern of cavernous malformations varies, the most common being a multifocal hyperintense center surrounded by a hypointense ring; 6) the MRI pattern reflects the histological appearance; 7) since no thrombosed arteriovenous malformations were encountered, such lesions must be rare; 8) in that the pathophysiological hallmark of a cavernous lesion is recurrent thrombosis and hemorrhage, a resolving hematoma cannot always be distinguished from a cavernous lesion; 9) MRI is the examination of choice in evaluating occult vascular malformations; and 10) microsurgical excision is a satisfactory method of treatment. Lastly, in that occult vascular malformations nearly always exhibit cavernous histology, we suggest the termcavernous angiomabe based upon the histological appearance of the vessels and that the somewhat artificial requirement of architectural compactness be abandoned, at least as far as central nervous system lesions are concerned.