A follow-up study was performed on 41 hydrocephalic children with primary (10 patients) or secondary (31 patients) placement of an anti-siphon device (ASD) connected to their shunt. The clinical course before and after ASD implantation was compared in these two groups, including studies of the head circumference, the radiological ventricular size, cerebrospinal fluid pressure studies on the supine and upright patient, and the history of shunt-induced complaints and complications leading to hospitalization. The authors discuss the pathophysiological concept of the "chronic overdrainage " or slit-ventricle syndrome based on ventricular collapse due to chronic suction-induced overdrainage by the vertical shunt system in the upright patient. The use of an ASD in connection with the regular shunt system of children with hydrocephalus is proposed. This ASD-shunt combination successfully reduced the number of complaints and shunt dysfunctions over a period of more than 6 years.