Hindbrain hernias with or without hydrosyringomyelia were difficult diagnostic problems before the availability of magnetic resonance imaging. Today, the problem seems not to be in evaluating the anatomical extent of the caudal herniation of the cerebellum, but in determining which patient should be considered for operative intervention and the extent of the surgery. Chiari I patients are presenting at younger ages, occasionally with irritability as their only symptom. Should all of these children be submitted to an operation? Chiari II patients are now operated on with the first detectable symptom or evidence of a syrinx, and yet medullary dysfunction from the Chiari II malformation remains the leading cause of death in treated myelomeningoceles today. Our knowledge of the natural history of the untreated conditions and the increased safety of the operation has made surgical intervention a much more viable option for this group of patients.