OBJECTIVE: Although there are many reports describing the risk of rebleeding in hospitalized patients with subarachnoid hemorrhage (SAH), it is common for these patients to deteriorate during transportation or while waiting for surgery. The aim of this study was to estimate the possible causes of neurological deterioration in patients with a ruptured aneurysm prior to hospitalization and management and the effect on patient prognosis. METHODS : Two hundred and thirty patients with aneurysmal SAH that arrived to the hospital within 24 hours after the initial bleeding were recruited. The course of neurological deterioration, age, gender, Hunt and Hess grade, Fisher grade, the arterial blood pressure, the clinical findings when neurological deterioration occurred, aneurysm location and size, treatment and the outcome were analyzed. RESULTS: Among the 230 patients, 32 (13.9%) patients had neurological aggravation prior to management. Fifteen (46.8%) patients had definite rebleeding, 5 (15.6%) acute hydrocephalus and 2 (6.3%) intracerebral hematomas. Two (6.3%) patients had a cardiac arrest, and another 8 (25%) patients likely had rebleeding. Such deterioration occurred in 15 (46.8%) patients within 3 hours after the initial event and in 21 (84%) patients within 6 hours. The patients experiencing neurological deterioration had a more severe Hunt and Hess grade, higher rates of intracerebral hematoma and intraventricular hematoma, reduced operability, and a poorer prognosis. CONCLUSION: The findings showed that neurological deterioration occurred much earlier than expected. Thus, more rapid and careful preoperative management and earlier neurosurgical intervention might prevent neurological deterioration and improve patient outcome.