In a series of 176 prospectively studied patients who survived for at least 24 hours after aneurysmal subarachnoid hemorrhage, 39 had at least one computerized tomography (CT)-proven rebleed within 4 weeks after the first rupture. There were peaks in the incidence of rebleeding at the end of the 2nd and 3rd weeks. Sudden loss of consciousness occurred in 35 patients, preceded in one-third of them by headache. A sudden increase in headache was a symptom of rebleeding in only one patient. Loss of brain-stem reflexes was recorded in 13 patients, respiratory arrest in six, and both symptoms in eight patients. Apnea was temporary in 11 patients. Rebleeding occurred as gross intraventricular hemorrhage in 20 patients, as a space-occupying hematoma in four, as both types of hemorrhage in three, and as a purely subarachnoid hemorrhage in 12. The location of the rebleed could not be inferred from the clinical features. Rebleeding was fatal in 51% of cases (two of 12 patients with a purely subarachnoid hemorrhage, and 18 of the other 27 patients (p less than 0.005)). The risk of rebleeding could not be predicted from the patients' clinical condition on admission or from the amount of subarachnoid blood identified on the initial CT scan. The risk of further rebleeding was significantly increased in survivors of a first rebleed (47%: p less than 0.01). Only seven (18%) of the 39 patients with rebleeding had survived at 3 months after the initial hemorrhage.