To compare the resolution rate of hydrocephalus after endoscopy (predominantly endoscopic third ventriculostomy [ETV]) using flexible endoscopes during a 5-year period in patients with a permeable and a nonpermeable subarachnoid space (SAS). We conducted a retrospective cohort study of the videos and records of 150 hydrocephalic patients chosen randomly who underwent ETV (and other endoscopic procedures) with a flexible endoscope. The patients were classified into two groups based on the neuroendoscopic findings. The first group included patients with a permeable SAS, and the second group included patients with a nonpermeable SAS. A normal SAS or one with slight arachnoiditis was considered permeable. Adhesive arachnoiditis and immature or mechanically obliterated SASs were considered nonpermeable. We found a success rate of 70% in patients with a permeable SAS versus 33% in patients with a nonpermeable SAS. The baseline characteristics of both groups were homogeneous. We obtained a statistically significant difference ( < 0.0001) with hazard ratio (HR) 3.42 (95% confidence interval [CI], 1.88-6.22). Another important factor involved was age that showed a statistically significant difference ( < 0.0018) with HR 3.28 (95% CI, 1.55-6.93). The permeability of the SAS is an important prognostic factor in the resolution rate of hydrocephalus after ETV (and other endoscopic procedures) using flexible neuroendoscopes. Therefore we recommend that the characteristics of the SAS be examined following every endoscopic procedure for hydrocephalus to identify patients at risk of recurrence.