The proper function of an antisiphon device (ASD) requires that it be exposed externally to atmospheric pressure and that its mobile membrane be free to move. A 4-year-old boy who had an ASD placed for postshunt subdural hematomas presented with functional obstruction of the ASD-symptomatic ventriculomegaly despite evidence of patency of the shunt system by radionuclide flow study. At surgery, the distal shunt system including the ASD was infused with normal saline before and after surgical exposure of the device. The pressures required to maintain a flow rate of 5 ml/h and 50 ml/h were 27 mm Hg and 30 mm Hg, respectively. After surgical exposure, these pressures fell to 0 mm Hg and 5 mm Hg, respectively, for the same infusion rates. Surgical exposure removes the effects of the tissue capsule, including the overlying skin and a collagenous sheath, restoring the external pressure to atmospheric pressure. Functional obstruction of ASDs occurs because of raised ambient pressure from the tissue capsule acting to depress the mobile membrane of the ASD, increasing its resistance to flow. This case confirms previously reported effects of subcutaneous implantation of ASDs in experimental animals and is the probable explanation for reported functional obstruction of ASDs in other patients.