When one mentioned about aneurysmal rebleeding, timing and incidence of the rebleeding attack (RA) have been mainly discussed in relation to long term prognosis. But it was rarely considered about necessity of emergency treatment based on timing and incidence of RA. In this study, clinical significance of RA especially in the early stage is discussed in relation to severity of the patient's condition, timing of surgery, clinical results and so on. Three hundred sixty-eight cases of anterior communicating, internal carotid and middle cerebral aneurysms who were admitted on our hospital within 7 days after last attack were studied. Among them, 127 cases had 192 times of RA (1 RA: 83 cases, 2 RAs: 28 cases, 3 RAs: 12 cases, 4 RAs: 3 cases and 5 RAs: 1 case). Incidence of rebleeding attack increased in the severe cases. Most frequent timing of second attack was within 6 hours after the first attack (40 cases, 31.5%). Fifty two cases (41%) had their second attack within 24 hours from the first attack. Successive attacks were also most frequent within 6 hours following the preceding one, even in the cases with long term interval period before it. Severity of the patient was increased after RA especially in the cases with early repetitive attack. We experienced 56 cases who had more than two RAs within each 6 hours. 28 out of 56 cases were severe attack whose consciousness was disturbed more than semicoma and 27 cases died. Mortality and morbidity were higher in the cases with RA than single attack.(ABSTRACT TRUNCATED AT 250 WORDS)