Sixty CT scans in 31 patients who underwent lumboperitoneal shunting for communicating hydrocephalus showed that the size of the ventricles did not represent a good indicator of shunt malfunction. Instead, we discovered that the size of the basal cisterns around the brain stem enabled us to predict blockage earlier and more reliably. In a well functioning shunt, the basal cisterns are usually not visualized. In children with clinical shunt malfunction the cisterns dilate and become visible again. This occurs earlier and more frequently than ventricular enlargement. We therefore conclude that visible cisterns in association with persistent symptoms of malfunction are more reliable predictors of a true blockage that requires shunt revision than serial studies of ventricular size.