Although many reports have described the rebleeding risk of the ruptured aneurysm in already hospitalized patients, there are only a few reports that have addressed the incidence of rebleeding in these patients before hospitalization. To improve the prognosis of patients with a ruptured intracranial aneurysm, it seems very important to know the incidence of rebleeding before hospitalization. We focused on changes in the computed tomographic (CT) scans and neurological findings at the primary hospital and our institution in 366 patients with ruptured aneurysms who were transferred on the day of the initial hemorrhage, and studied the rebleeding rates in these patients. In 73 (19.9%), we confirmed that rebleeding from the ruptured aneurysm had occurred during transport. The incidence of rebleeding in the prehospitalized patients with a ruptured aneurysm is supposed to by very high. Appropriate medical countermeasures for prevention of rebleeding in prehospitalized patients are crucial to decrease the overall mortality and morbidity rate of intracranial aneurysm.