A simple method is presented for detecting the patency of ventriculoperitoneal shunts with a pair of small disc thermistors. In an experimental model, the equipment detected definite temperature changes along the shunt tube. The extent of the temperature difference was proportional to the flow velocities, ranging from 5 to 60 ml/hr (120 to 1440 ml/24 hrs). A clinical study consisted of 26 trials in 23 hydrocephalic children and 10 trials in nine adult patients with normal-pressure hydrocephalus. Three of the 26 trials in the children were unsuccessful because of patient irritability and lack of cooperation. All trials for adult patients were successful. In all 25 patients, whose clinical pictures suggested functioning shunt, accurate confirmation of shunt patency was obtained. In six patients whose clinical pictures suggested questionable shunt function, this method revealed that all shunts were patent, and computerized tomography and follow-up clinical data supported this finding. In two patients whose clinical pictures suggested shunt malfunction, there was no indicator deflection, and shunt obstruction was proved at operation. This method is effective in checking shunt function, is simple, and may be repeated at frequent intervals.