PURPOSE: To determine how clinical and angiographic factors relate to the amount of subarachnoid blooddetected by computerized tomography in patients with a ruptured aneurysm. MATERIALS AND METHODS: Between January1996 and December 1997, 22 patients with a posterior communicating artery aneurysm were retrospectively evaluated. RESULTS: Oval(three of four cases), funnel(both cases), and daughter-sac (four of five cases) types of aneurysmalsac were found among the 13 patients with a large amount of subarachnoid blood ; eight of these had a past historyof hypertension or diabetes. Seven of eleven cases of cylindrical-type aneurysmal sac were found among the 9patients with a small amount of sularachnoid blood ; eight of these had no past history of hypertension ordiabetes. The average S/N ratio (ratio of maximum sac length to neck diameter) of patients with a small amount ofblood was higher than that of patients with a large amount of blood(2.72 vs 2.07). CONCLUSION: Although manyfactors influence the amount of subarachnoid blood in an aneurysmal rupture, we found that a large amount of bloodwas frequently present in the oval, funnel and daughter sac types of aneurysm, when S/N ratio was low, and when anunderlying disease such as hypertension or diabetes was present. Conversely, a small amount of blood was presentin the cylindrical type, when S/N ratio was high, and where there was no of underlying disease.