From 1986 to 1989, 23 adult patients (average age, 70 yr) with idiopathic chronic hydrocephalus received shunts with medium-pressure Pudenz-Schulte valves for suspected normal pressure hydrocephalus. Prospective clinical and computed tomographic monitoring was continued for at least 5 years. We observed the formation of a hypodense subdural collection in each of 10 patients (43%). Those collections that occurred early, i.e., within the first 9 postoperative days, evolved differently from those that occurred late; only early hypodense collections became subdural hematomas (three cases). In one case, a subdural hematoma was already present 9 days after surgery, so that four patients (17%) presented a subdural hematoma within the first 2 postoperative months. Our long-term follow-up revealed three patients (13%) with hypodense subdural collections, which appeared more than 2 months after surgery. None of the collection evolved into a subdural hematoma. Thirteen patients (57%) died between 9 and 68 months (average, 20 mo) after surgery, most often of an ischemic stroke. During the 1st postoperative year, there was improvement in the condition of 22 patients (96%) who had received a ventricular shunt; 21 of these patients (91%) remained improved until death or for at least 5 years.