Acquired tonsillar herniation and syrinx formation are recognized complications of a lumboperitoneal shunt used to treat conditions of increased CSF volume. Treatment of this complication is by ventriculoperitoneal shunt or foramen magnum decompression. We describe the first case of acquired tonsillar herniation in a pseudotumour cerebri (following lumboperitoneal shunt insertion) that responded partly to ventriculoperitoneal shunt insertion and more completely to foramen magnum decompression. Our case demonstrates that a ventriculoperitoneal shunt is not always sufficient in treating this complication and reversing visual deterioration. Foramen magnum decompression may therefore be a sight saving procedure in pseudotumour cerebri with acquired tonsillar herniation and may be important in understanding the pathogenesis of conditions of increased CSF volume.