BACKGROUND AND PURPOSE: Endoscopic third ventriculostomy is a well-accepted treatment choice for hydrocephalus and is used most frequently with a known impediment to CSF flow between the third ventricle and basal cisterns. However, there are scarce data on the imaging evolution of the defect in the floor of the third ventricle and how this affects patency rates and clinical outcomes. The purpose of this study was to assess whether, and how, the endoscopic third ventriculostomy defect changes in size with time. MATERIALS AND METHODS: All high-resolution endoscopic third ventriculostomy protocol MRIs performed between 2009 through 2014 were retrospectively identified. Two fellowship-trained neuroradiologists, blinded to clinical information, independently reviewed all retrospective cases. RESULTS: A total of 98 imaging studies were included from 34 patients. The average change in the area throughout the studied period was 0.02 mm 2 /day (7.5 mm 2 /year), with a higher increase in size noted in the first 3 postsurgical months, with a gradual decrease in the degree of defect-size change. Use of the NICO Myriad device was correlated with the area of the endoscopic third ventriculostomy defect on the last follow-up, demonstrating a larger final defect size in patients in whom the surgical technique included debridement of the endoscopic third ventriculostomy defect walls with the NICO Myriad device (28.21 versus 11.25 mm, P .05). CONCLUSIONS: High-resolution MR imaging with sagittal CISS images is useful in the postoperative evaluation of endoscopic third ventriculostomies. Such findings may prove useful in determining the optimal duration of follow-up with MR imaging of patients who have undergone endoscopic third ventriculostomy.