OBJECTIVE The optimal surgical treatment of Chiari malformation is unclear, especially in patients with hydromyelia. Various surgical approaches have included suboccipital craniectomy, syringostomy, obex plugging, syringosubarachnoid shunting, and fourth ventriculosubarachnoid shunting. The purpose of this study is to differentiate extradural and intradural approaches in the treatment of Chiari I malformation. METHODS We reviewed the medical records and magnetic resonance imaging (MRI) scans of 34 surgical corrections' of Chiari malformation performed at our institution from 1988 to 1998. The age and sex of the patient, the presence of hydromyelia, the type of surgery (duraplasty or nonduraplasty), and the clinical outcome were determined. RESULTS Eleven patients underwent posterior fossa decompression (PFD) and C1 laminectomy without duraplasty. Eight (73%) of these patients had an improvement in symptoms. Seven of the 11 patients had hydromyelia. Of the six patients who underwent follow-up MRI, three (50%) had a decrease in the size of the hydromyelia, and all three had clinical improvement. We also noted a morphometric increase in posterior fossa volume on postoperative MRI scans in these three patients, which was not observed in those without improvement. Two of the three patients whose hydromyelia did not decrease on follow-up MRI scans worsened clinically, and one underwent a reoperation with duraplasty. Twenty-three patients underwent combined PFD, C1 laminectomy, and duraplasty. Twenty (87%) of these patients had improvement. Twelve of the patients who underwent duraplasty had hydromyelia; nine underwent follow-up MRI. All nine of these patients (100%) had a decrease in the cavity size, including eight with clinical improvement. There were 10 minor complications (seroma, 4; superficial infection, 3; cerebrospinal fluid leak, 2; aseptic meningitis and occipital nerve pain, 1) when the dura was opened, compared with one superficial wound infection that resolved in patients who underwent PFD only. CONCLUSIONS PFD, C1 laminectomy, and duraplasty for the treatment of Chiari I malformation may lead to a more reliable reduction in the volume of concomitant hydromyelia, compared with PFD and C1 laminectomy alone. However, there seems to be a subset of patients whose symptoms will resolve and whose hydromyelic cavity will decrease with the removal of bone only. These patients seem to undergo a volumetric increase in the posterior fossa. Further studies are needed to better characterize these patients, to determine which patients with Chiari I malformation are better served with bony decompression only, and which will require duraplasty to resolve their hydromyelia.