OBJECTIVE: Earlier reports have revealed that the incidence of posttraumatic hydrocephalus (PTH) is higher among patients who underwent decompressive craniectomy (DC). The aim of this study was to determine the influencing factors for the development of PTH after DC. METHODS: A total of 693 head trauma patients admitted in our hospital between March 2004 and May 2007 were reviewed. Among thee, we analyzed 55 patients with severe traumatic brain injury who underwent DC. We excluded patients who had confounding variables. The 33 patients were finally enrolled in the study and data were collected retrospectively for these patients. The patients were divided into two groups: nonhydrocephalus group (Group I) and hydrocephalus group (Group II). Related factors assessed were individual Glasgow Coma Score (GCS), age, sex, radiological findings, type of operation, re-operation and outcome. RESULTS: Of the 693 patients with head trauma, 28 (4.0%) developed PTH. Fifty-five patients underwent DC and 13 (23.6%) developed PTH. Eleven of the 33 study patients (30.3%) who had no confounding factors were diagnosed with PTH. Significant differences in the type of craniectomy and re-operation were found between Group I and II. CONCLUSION: It is suggested that the size of DC and repeated operation may promote posttraumatic hydrocephalus in severe head trauma patients who underwent DC.