The authors studied the acute neurological deterioration in 150 patients with aneurysmal hemorrhage. These patients were closely observed for a 4-week period or until death or operation. Forty-six patients suffered a total of 62 episodes of deterioration that developed within 5 minutes after aneurysm rupture; 59 of these episodes consisted of severe impairment of consciousness. Computerized tomography (CT) was performed within a few hours of the event. Serial scans were obtained at least weekly after admission. Rebleeding was diagnosed as the cause of 42 episodes (68%), including six patients who died rapidly without a repeat CT scan. The final diagnosis of the cause in other acute episodes was epilepsy (in six), acute onset of ischemia (in three), and ventricular fibrillation (in one). Ten unexplained events occurred in nine patients. Rebleeding could be excluded with confidence in nine of these events, because the residual clots had disappeared or markedly decreased on CT. It is concluded that a purely clinical diagnosis of rebleeding will be incorrect in about every third patient, even if these patients are under close observation.