During the past seven years, we have studied 661 cases of ruptured intracranial aneurysms. Rebleeding occurred in 65 cases (10%) and, within this group, 43 cases (70%) rebled within the first 6 hours after initial subarachnoid haemorrhage (SAH). Analysis of these 43 cases led to the following conclusions: 22 patients incurred rebleeding from causes such as transfer (6 cases), neuroradiological examinations (13 cases), and tracheal intubation during anaesthesia etc. (3 cases), while no special causative factors were discovered in the other 21 cases. Rebleeding occurred in 19 patients even while on absolute bed rest and in 11 patients who had induced systemic arterial hypotension (under 140 mmHg) through treatment. Six cases experienced rebleeding while undergoing angiography within 6 hours after the first subarachnoid haemorrhage. Eight of 17 reruptured anterior cerebral complex (Acom) aneurysm cases and 8 of 11 reruptured middle cerebral artery (MCA) aneurysm cases had an intracerebral haematoma on initial CT-scan following the first attack, demonstrating that the risk of rebleeding was very high in cases of intracerebral haematoma. The mortality rate for these rebleeding cases was high i.e. 65%. Therefore, because the time factor could precipitate rebleeding, early transfer and operation was considered optimal for minimizing rebleeding soon after an aneurysm rupture, even though angiography within 6 hours of the first SAH was a serious risk. Barbiturate therapy, performed as early as possible for serious cases, was considered to be effective in preventing rebleeding.