Objective To investigate the risk factors of posttraumatic hydrocephalus (PTH) in patients with moderate to severe traumatic brain injury (TBI).Methods Aretrospective study was conducted for 183 patients with moderate to severe TBI (125 males,58 females;6-91 years of age,mean 48.23 years).According the presence of PTH,the patients were allocated into PTH group (n =34) and non-PTH group (n =149).Risk factors of PTh were assessed by univariate and logistic regression analysis,including gender,age,injury types,injury severity,intraventricular hemorrhage,subarachnoid hemorrhage,midline shift,subdural effusion,therapeutic strategies and skull defect.Association between the boundaries of skull defect and PTH was determined.Results Between-group differences were not significant regarding age,gender,injury types and intraventricular hemorrhage (P ＞ 0.05),but differed significantly in injury severity,subarachnoid hemorrhage,midline shift,subdural effusion,craniectomy and skull defect (P ＜ 0.05).Further Logistic regression analysis confirmed subarachnoid hemorrhage (OR =6.169),interhemispheric subdural effusion (OR =31.743),and unilateral (OR =17.602) and bilateral (OR =30.567) skull defects were risk factors of PTH.Of the patients with unilateral skull defect following decompressive craniectomy,the inferior limit ≤ 10 mm from the zygomatic arch also played a role in the development of PTH (OR =5.500,P ＜ 0.05).Conclusions Subarachnoid hemorrhage,interhemispheric subdural effusion and skull defect are risk factors of PTH.Unilateral skull defects with the inferior limit too close to the zygomatic arch can predispose to the development of PTH.