The case histories of 38 children with implanted antisiphon devices (ASD) were retrospectively reviewed. Sixty-three percent suffered a shunt malfunction within 5 years of ASD implantation. The probability of shunt malfunction within 1 year was of 40%. Patients with myelomeningocele experienced the highest rate of malfunction. Age, order of ASD implantation (whether implanted primarily or secondarily), shunt pressure, and reason for ASD insertion did not significantly alter the incidence of malfunction. Fifty percent of the shunt complications were specifically related to ASD malfunction. Functional obstruction of the shunt system as determined by evidence of clinical shunt obstruction with documented patency on shunt flow testing occurred in 10 of 38 patients. No variable studied could be significantly related to the probability of developing an ASD-related complication. Functional obstruction of the ASD is a common and unpredictable complication of these devices.