Radionuclide quantitation of cerebrospinal fluid (CSF) flow through a ventricular shunt using a Cordis-Hakim valve was performed in 78 adults (138 studies). All remained supine for at least two hours before the study to avoid possible depletion of ventricular CSF. The absolute flow rate distinguished shunt adequacy from patency by defining flow of less than 0.1 ml/min. as abnormal. Measurements were performed with the patient first supine and then erect. When abnormal supine flow was the criterion of malfunction, all obstructions were detected but there were 6 false positives. When abnormal erect flow was considered, there were 5 false negatives. Abnormal flow in both supine and erect positions is a reliable indication of malfunction. The authors conclude that this technique is valuable for neurosurgical management of ventricular shunts.