The medical records of 30 patients with histologically confirmed and angiographically occult intracranial vascular malformations(AOVM), who underwent surgery between May 1988 and May 1993, were reviewed retrospectively to determine whether their radiological and clinical characteristics are helpful in differential diagnosis. Histological diagnoses were cavernous angioma(CA) in 17 cases, arteriovenous malformation(AVM) in nine, venous angioma in one and unclassified vascular malformation in three. The most common initial presenting mode was intracranial hemorrhage(ICH ; 18 cases, 60.0%), followed by seizure(11 cases, 33.3%) and headache(two cases, 6.6%). CA, once it had bled, tended to bleed repeatedly, and this occurred before surgery in seven of nine cases of CA presenting with ICH. On CT scan, calcification was observed only in CA(two cases). On MRI images obtained in 28 patients, a mottled density mass with or without adjacent ICH(ten of 16 CA's) and multiple lesions(three of 16 CA's) were pathognomonic for CA, while single stage ICH(two of eight AVM's) and signal void(three of eight AVM's) were observed only in cases of AVM. Findings of MRI such as multiple stage hemorrhage, low signal intensity rim or edema around the lesion were not helpful in differential diagnosis of the histological type of lesions. After enhancement with gadolinium, one case of AVM and another of venous angioma showed a serpentine pattern of enhancement. In 29 cases, the results of surgery were excellent ; there was no mortality and morbidity in only one case. In conclusion, CA, once it had bled, tended to rebleed and MRI was helpful in the differential diagnosis of AOVM's. MRI findings such as a mottled density mass or multiple lesions were pathognomonic for CA, while single stage hemorrhage or signal void were findings of AVM.