OBJECT: The object of this study was to analyze the outcome of endoscopic third ventriculostomy (ETV) in patients under 2 years of age and investigate factors related to ETV success or failure in this patient population. METHODS: The authors reviewed their experience in using endoscopic third ventriculostomy (ETV) in the treatment of 41 hydrocephalus patients younger than 2 years. The mean duration of follow-up was 45 months. The relationship between ETV efficacy and the following variables was analyzed: cause of hydrocephalus, level of CSF occlusion, primary versus secondary ETV, type of surgical procedure, head circumference, patient age at ETV, patient age at first manifestation of hydrocephalus, and anatomical features of the ventricle. Success of ETV was assessed based on the results of neurological examination and postoperative imaging during the follow-up period. RESULTS: The authors performed 32 primary ETVs and 10 secondary ETVs (ETV after hydrocephalus surgery) in 41 patients (a second ETV was performed in 1 patient). The success rates of primary and secondary ETV were 75.8 and 55.6%, respectively (no significant difference, p = 0.15). The ETV was clinically and radiologically successful in 30 (71.4%) of 42 procedures during a mean (+/- SD) follow-up period of 45.0 +/- 4.8 months (range 12-127 months). A negative relationship was found between success of ETV and the thickness of the floor of the third ventricle (the most effective procedures were those in which the floor of the ventricle was thinnest [p