OBJECTIVE At failure of endoscopic third ventriculostomy (ETV) with choroid plexus cauterization (CPC), the ETV ostomy may be found to be closed or open. Failure with a closed ostomy may indicate a population that could benefit from evolving techniques to keep the ostomy open and may be candidates for repeat ETV, whereas failure with an open ostomy may be due to persistently abnormal CSF dynamics. This study seeks to identify clinical and radiographic predictors of ostomy status at the time of ETV/CPC failure. METHODS The authors conducted a multicenter, retrospective cohort study on all pediatric patients with hydrocephalus who failed initial ETV/CPC treatment between January 2013 and October 2016. Failure was defined as the need for repeat ETV or ventriculoperitoneal (VP) shunt placement. Clinical and radiographic data were collected, and ETV ostomy status was determined endoscopically at the subsequent hydrocephalus procedure. Statistical analysis included the Mann-Whitney U-test, Wilcoxon rank-sum test, t-test, and Pearson chi-square test where appropriate, as well as multivariate logistic regression. RESULTS Of 72 ETV/CPC failures, 28 patients (39%) had open-ostomy failure and 44 (61%) had closed-ostomy failure. Patients with open-ostomy failure were older (median 5.1 weeks corrected age for gestation [interquartile range (IQR) 0.9–15.9 weeks]) than patients with closed-ostomy failure (median 0.2 weeks [IQR -1.3 to 4.5 weeks]), a significant difference by univariate and multivariate regression. Etiologies of hydrocephalus included intraventricular hemorrhage of prematurity (32%), myelomeningocele (29%), congenital communicating (11%), aqueductal stenosis (11%), cyst/tumor (4%), and other causes (12%). A wider baseline third ventricle was associated with open-ostomy failure (median 15.0 mm [IQR 10.3–18.5 mm]) compared to closed-ostomy failure (median 11.7 mm [IQR 8.9–16.5 mm], p = 0.048). Finally, at the time of failure, patients with closed-ostomy failure had enlargement of their ventricles (frontal and occipital horn ratio [FOHR], failure vs baseline, median 0.06 [IQR 0.00–0.11]), while patients with open-ostomy failure had no change in ventricle size (median 0.01 [IQR -0.04 to 0.05], p = 0.018). Previous CSF temporizing procedures, intraoperative bleeding, and time to failure were not associated with ostomy status at ETV/CPC failure. CONCLUSIONS Older corrected age for gestation, larger baseline third ventricle width, and no change in FOHR were associated with open-ostomy ETV/CPC failure. Future studies are warranted to further define and confirm features that may be predictive of ostomy status at the time of ETV/CPC failure.