OBJECTIVE MRI volumetry could be used as an alternative to invasive tests of shunt function. In this study, the authors aimed to assess the difference in ventricular volume (VV) before and after surgery and at different performance levels (PLs) of the shunt. METHODS This study was a randomized, double-blind trial with a crossover design. The study sample consisted of 36 patients (25 men, 11 women) with a median age of 76 years. All patients had idiopathic normal pressure hydrocephalus (iNPH) and received a Strata shunt at the regional hospital in & Ouml;stersund, Sweden, with an initial PL of 1.5. Participants underwent MRI with volumetric sequences before surgery and four times postoperatively: at 1 month before randomization to either PL 1.0 (n = 15) or 2.5 (n = 17); at 2 months before crossover to PL 2.5 or 1.0; at 3 months before lowering the PL to 0.5; and finally, at 3 months and 1 day after surgery before resetting the PL to 1.5. VV was measured semiautomatically using quantitative MRI. Both the patient and the examiner of clinical tests and volumetry were blinded to the PL. RESULTS VV changed significantly between the presurgical level (median 129 ml) and the different shunt settings, i.e., PL 1.0 (median 115 ml), 1.5 (median 120 ml), and 2.5 (median 128 ml; p < 0.001). A unidirectional change in VV was observed for all participants between PL 1.0 and PL 2.5 (median 12 ml, range 2.1-40.7 ml, p < 0.001). No significant change was noted in VV after 24 hours at PL 0.5. Eight participants had asymptomatic subdural effusions at PL 1.0. CONCLUSIONS The consistent decrease in VV after shunt surgery and between PL 2.5 and 1.0 supports the idea that MRI volumetry could be a noninvasive method for evaluating shunt function in iNPH, preventing unnecessary shunt revisions. However, further studies on retest variability of VV as well as verification against advanced testing of shunt function are needed before a clinical implementation of this method can be performed.