Reduction in ventricular size after third ventriculostomy often appears minimal. In order to determine if changes in ventricular size are a reliable measure of a functioning ventriculostomy, and which ventricle is more sensitive to this change, we compared pre-operative lateral and third ventricular diameter with serial post-operative axial CT and/or MRI scans in a series of 27 consecutive third ventriculostomies. Of eleven patients with aqueductal stenosis, fifteen with mass lesions and one with a Chiari I, six patients were shunted before 1 month and two refused radiographic follow-up. In the remaining nineteen patients, with an average follow-up of 20 months, the average (s.d.) third ventricular change was -30 (11)% for twelve improved patients, and +13 (18)% for the seven patients without improvement (p < 0.00001). The average (s.d.) lateral ventricular change was -23 (15)% for improved patients, and +5 (21)% for those without improvement (p < 0.005). A decrease in third ventricular size by a least 15% was seen in all significantly improved patients. Similar results were found in 9 patients with scans performed exactly one month post-operatively. Among the aqueductal stenosis patients, factors associated with failure were age > 70 years associated with symptoms of NPH, symptomatic, shunted hydrocephalus since before age 1 year and a history of meningitis. In the remaining patients, failures occurred with a history of meningitis, subdural hematoma, Chiari I malformation or following open tumor surgery or intraventricular AVM rupture. Although both the lateral and third ventricles change in diameter after third ventriculostomy, the third ventricular diameter is more reliable. As early as one month post-operatively, ventricular diameter reflects the clinical outcome of the operation. A decrease in third ventricular diameter by at least 15% was seen in all successful cases and in none of the failures.