Posthemorrhagic hydrocephalus requiring permanent ventriculoperitoneal shunt placement is a major complication of aneurysmal subarachnoid hemorrhage (SAH). High S100B serum and cerebrospinal fluid (CSF) levels are considered to reflect the severity of brain injury. We prospectively assessed whether S100B levels in serum and CSF were predictive parameters for permanent shunt requirement following aneurysmal SAH. In patients suffering from aneurysmal SAH and treated with an external ventricular drain (EVD), S100B levels in serum and CSF were measured daily as long as the EVD was in place. S100B levels of patients who passed their EVD challenge were compared with those patients who required a permanent ventriculoperitoneal shunt placement. Out of 68 patients included in the study, 43 patients (63.2%) passed the EVD challenge and in 25 patients (36.8%) permanent ventriculoperitoneal shunting was performed. Group comparison revealed that in patients who required shunt placement, S100B was significantly higher in CSF (p < 0.05 at days 2, 4, 6, 10; p < 0.005 at days 1, 3, 5, 7, 8, 9) and serum (p < 0.05 at days 4-7) compared with patients who could be weaned from the EVD. Assessment of S100B levels in CSF and serum may be useful as a predictive parameter for shunt dependency in patients with posthemorrhagic hydrocephalus following aneurysmal SAH.