In the 5-year period from 1978 to 1983, 1076 patients with ruptured intracranial aneurysms were admitted to the six neurosurgical departments in Denmark and were entered in a prospective consecutive study conducted by the Danish Aneurysm Study Group. The patients were followed with 3-month and 2-year examinations or to death. A total of 133 patients suffered at least one rebleed after their initial hemorrhage during their first stay in the neurosurgical department; these patients had a mortality rate of 80% compared to 41% for patients without a rebleed (p less than 0.0001). During the first 2 weeks after the initial insult, 102 rebleeds were registered. The daily rate of rebleeds during these 2 weeks, calculated using a life-table method, varied from 0.2% to 2.1%. The rebleed rate during the first 24 hours (Day 0) was 0.8%, and the maximum risk of rebleeding was observed between Day 4 and Day 9. Significantly fewer rebleeds were reported in patients with good clinical grades (Grades 1 to 3, Hunt Grades I and II) compared to those with poor clinical grades (Grades 4 to 9, Hunt Grades III to V: p less than 0.001).