240 patients with giant aneurysms admitted to 10 Italian centres were evaluated regarding radiological features and relationships to the outcome. Visualization of the aneurysm without contrast was obtained on CT scan in 49% of patients with aneurysms between 2 and 2.5 cm in diameter (A 1 group) and in 80% of patients with larger aneurysms (A 2 group). Contrast enhancement was homogeneous in 49% of patients, not homogeneous--with central or peripheral hypodensity--in 47% of patients, absent in 4% of patients. Ventricular shift was present in 17% of A 1 patients and in 36% of A 2 patients. Hypodense areas were observed in 12% of cases, and calcifications in 19% of cases; bone erosions were rare. On angiography, the most common aneurysmal locations were the intracavernous carotid (21% of cases) and the middle cerebral artery (23% of cases). A neck could be identified on angiography in only 14% of patients, and stenosis of afferent vessel in 16% of patients. Vasospasm was rarely observed on angiography (17% of A 1 and 10% of A 2 patients). Aneurysmal thrombosis (partial, subtotal or total) was present in 48% of A 1 and 76% of A 2 patients (p less than 0.001); partial peripheral thrombosis was most commonly observed (one half of cases). Round or oval shapes were most commonly observed, while fusiform, irregular, serpentine or doughnut shapes were rare. A few clinical/radiological relations were considered, such as the relation between partial thrombosis and haemorrhage, symptoms of expanding mass lesion, ischaemic episodes, aneurysmal location.(ABSTRACT TRUNCATED AT 250 WORDS)