Abstract Objective: Endoscopic third ventriculostomy (ETV) is considered to be a safe and effective treatment in selected patients as an initial treatment for obstructive hydrocephalus and at the time of shunt malfunction in previously shunted patients. We compared the outcome and complications of ETV between patients with newly diagnosed hydrocephalus and those with previous shunting procedures. Methods: A retrospective review of patients undergoing ETV from 1996 to 2004 at Alberta's Childrens Hospital and Foothills Medical Centre was completed. Patient data included symptoms at clinical presentation, cause of hydrocephalus, age at initial shunt, number of previous shunt revisions, age at ETV, complications, and subsequent shunting procedures performed. Results: A total of 131 patients were identified with a minimum follow-up duration of 1 year; 71 (82.5%) of 86 patients who underwent ETV as a primary procedure and 36 (80%) of 45 patients who had ETV at the time of shunt malfunction were shunt-free at the last follow-up evaluation. Patients younger than 1 year old who underwent ETV were more likely to require an additional procedure for control of their hydrocephalus (P< 0.01). Serious complications after ETV occurred more frequently in patients who presented at the time of shunt malfunction (14 of 45 patients, 31%) compared with patients who underwent primary ETV (seven of 86 patients, 8%) (P = 0.02). Previously shunted patients with a history of two or more revisions (P= 0.03) and who experienced a serious complication at the time of ETV (P = 0.01) were more likely to require shunt replacement. Conclusion: ETV is an effective treatment both in selected patients with newly diagnosed hydrocephalus and in patients with a previous shunting procedure who are presenting with malfunction. Complications of ETV occur more frequently in previously shunted patients than in patients treated for newly diagnosed hydrocephalus, and care must be taken in the selection and treatment of these patients.