The Chiari I malformation is associated with a hydromyelic cavity in 55-75% of patients. In the author's series (1977-1991), hydromelia occurred in 111 of 131 patients (85%). There were 35 children. Posterior fossa decompression with duraplasty and fourth ventricle to subarachnoid shunt was made in all patients with Chiari I malformation with hydromyelia. The obex connection to the cervical cord central canal was closed with muscle, if widely patent. Eighty-five percent showed sustained objective improvement. The precautions taken to avoid complications and a literature review are tabulated for easy reference.