Considerable debate exists about which surgical options are best for the management of the Chiari I malformation. We present a classification system for the Chiari I malformation that improves the prediction of outcome and guides the selection of surgical treatment. Twenty-seven adult patients with Chiari I malformations were grouped on the basis of the presence of signs and symptoms of brain stem compression, syringomyelia, or both. To objectively assess changes in clinical status postoperatively, a scale was developed to quantify the signs and symptoms, which were statistically analyzed by the paired t test. Five patients were asymptomatic and underwent no treatment. Ten patients had symptoms of brain stem compression without associated syringomyelia and underwent brain stem decompression, including anterior decompression in one patient with basilar invagination; all 10 patients had significant improvement at 4-year mean follow-up visits (P < 0.0001). In 12 patients with syringomyelia, 5 were symptomatic from syringomyelia only, 6 were symptomatic from both brain stem compression and syringomyelia, and 1 was symptomatic from brain stem compression only. The median length of symptoms before presentation was longer for patients with syringomyelia than for patients without (2 yr versus 9 mo; P < 0.025); the mean follow-up was 4 years. Surgical procedures included posterior brain stem decompression in 12 patients, plugging of the obex in 7, and placement of syringosubarachnoid shunts in 7, a syringopleural shunt in 1, and fourth ventricular stents in 2. In the 12 patients with syringomyelia, symptoms from brain stem compression dramatically improved with surgical decompression (P < 0.025), whereas symptoms from syringomyelia less dramatically improved or stabilized. The slight improvement or stabilization of syrinx symptoms represents a successful result, given the documented progressive nature of syringomyelia in this group. We conclude that surgical treatment for the Chiari I malformation can stabilize or slightly improve the symptoms attributed to syringomyelia and dramatically relieve the symptoms of brain stem compression. Furthermore, early diagnosis and treatment are critical in obtaining the best outcome for the patient.