The differences between initial bleeding and rebleeding due to ruptured intracranial aneurysm was investigated by CT. The study included 120 cases of ruptured cerebral aneurysm; 71 cases had only one bleeding and 49 cases had rebleeding. All the cases underwent CT examinations within 2 weeks following bleeding. Subarachnoid hemorrhages were classified into three groups according to the CT findings: Diffuse high type--high density areas in the entire basal cistern; Localized high type--high density area in a restricted part of the basal cistern; and Iso or Low type--no high density areas in the basal cistern. CT scans done within 24 hours following initial bleeding showed Diffuse high type in 96% of the cases and Localized high type in 4%. On the other hand, in rebleeding cases, Diffuse high type was 50%, Localized high type 25% and Iso or Low type 25%. In rebleeding cases within three days following initial bleeding, Diffuse high type was 90% and Iso or Low type 10%. However, between day 4 and day 14, Diffuse high type was 14%, Localized high type 42% and Iso or Low type 42%. After day 15, Diffuse high type was 36%, Localized type 36% and Iso or Low type 28%. Intracerebral hematoma and/or ventricular hemorrhage secondary to ruptured aneurysms were also investigated. Following initial bleeding, 18% of cases showed intracerebral and/or ventricular hemorrhage, however, the incidence increased up to 59% in rebleeding cases.(ABSTRACT TRUNCATED AT 250 WORDS)