Three patients are described in whom syringomyelia was identified long after the treatment of communicating hydrocephalus by a lumboureteral shunt. The reason for syrinx formation in these cases could not be determined. In two there was either autopsy-proven or presumed evidence for arachnoiditis, and in the third patient the symptoms of syringomyelia were acutely aggravated by temporary obstruction of shunt. The development of a pressure drop from the intracranial compartment to the spinal compartment with crowding at the foramen magnum is also a suggested mechanism.