Chiari malformation is a condition characterized by herniation of the posterior fossa contents below the level of the foramen magnum : The main pathologic change is downward displacement of the cerebellar tonsils to occlude the subarachnoid space at the level of the foramen magnum. For this reason, the practice of posterior fossa decompression in the treatment of syringomyelia with Chiari malformation has been widely accepted. In order to evaluate the usefulness of the procedure in the treatment of this condition, clinical data and surgical outcome in 20 patients who underwent posterior fossa decompression during the last five years were analyzed. The average age at presentation was 35.9(range 17 to 61) years. Chiari I malformation was found in 14 patients, and Chiari II malformation in six : weakness and pain were the most common symptoms. The most useful preoperative imaging study was magnetic resonance imaging. In order to expose the outlet of the fourth ventricle, all patients underwent suboccipital craniectomy and C1 or C1-2 laminectomy with the supportive procedure consisting of adhesiolysis and tonsillar elevation. Postoperatively, 75% of patients showed improvement, and the condition of 15% stabilized. Postoperative MRI was performed in 12 patients, and revealed that in 11 of these, the syrinx cavity had become smaller. Transient headache and vomiting was the most frequent postoperative complication. Patients with pain and numbness had a better prognosis than those with weakness and sensory loss, and it appears that in cases of syringomyelia with Chiari malformation, posterior fossa decompression is beneficial.