Computed Tomograhy has become increasingly important diagnostic method as the initial examination in the diagnosis of subarachnioid hemorrhage with direct detection of extravasated blood in basal cistern and cortical sulci.Furthermore, CT provides better and exact visualization of the presence, localization , extent and degree of intracerebral , intraventricular and subdural hemorrhage, infarction, hydrocephalus an drebleeding whcih may be associated with subarachnoid hemorrhage, and also could detect the causative lesions with contrast enhancement in many cases. The purpose of the paper is to describe the CT findings of subarachnoid hemorrhage due to various causes and to evaluate the diagnostic accuracy of CT in subarachnoid hemorrhage. Authors analysed a total of 153 cases with subarachnoid hemorrhage confirmed by lumbar puncture at Seoul National University Hospital from Marhc 1979 to April 1981, with special emphasis on CT findings. All of the cases took CT scan and 125 ases of them angiography. The results are as follows : 1. Most prevalent age group was 4th to 6th decades(78%) . The ratio of male to female was 1.1 : 1. 2. Of 125 cases with angiography , aneurysm was a major cause(68%). Others were arterio-venous mal-formation (9.6%), Moya -moya disease (4%) and unknown(18.4%). 3. Of all 153 cases with CT scan , hemorrhage was demonstrated in 98 cases (64.1%) ; SAH in 72 cases (47.1%), ICH in 65 cases(42.5%), IVH in 34 cases(22.2%) and SDH in 1 cases (0.7%). SAH combined with ICH was a major group (34.7%) in SAH. Detection rate of SAH was 68.3% within the first 7 days and 5.8% after 7 days. 4. In aneurysms, SAH was detected in 60 of 85 cases(70.6%) ; 88.1% within the first 7 days and 5.6% after 7 days. Anterior communicating artery was the most common site of the aneurysms (40%), in which detection rate of SAH was 100% within the first 7 days. SAH was combined with ICH in 38.3%. 5. On CT , SAH of unilateral Sylvian fissure was pathognomonic for rup ured MCA aneurysm and ICH of corpus callosum for ACA aneurysm. 6. The detection rate of aneurysm itself on CT 20% 917/85) and its size was variable form 5 to 25 mm. 7. Infarction was demonstrated in 9 cases(6%) and there was no significant correlation between infarction and angiospasm. 8. Hydrocephalus was detected in 65 cases(42.5%), and 21 cases(32.3%) were demonstrated within the first 3 days.