We analyzed the outcome of posterior fossa decompression accompanied by widening of the cisterna magna, without disturbing the arachnoid, in patients with Chiari I malformation (CMI) associated with syringomyelia. Twenty-five adult patients with CMI and syringomyelia, who underwent surgery between October 2000 and December 2008, were enrolled in this study. All patients underwent foramen magnum decompression with C1 decompression, with or without C2 decompression. Three surgeons performed a dura opening with duraplasty in 20 patients, and another surgeon excised the outer layer of the dura without duraplasty in five patients. Clinical and radiological assessments were performed preoperatively and during the follow-up period. After surgery, 20 (80%) patients achieved a significant improvement in their clinical symptoms. However, four patients (16%) achieved only a stable state, and one patient's symptoms worsened. Radiological analysis showed that 17 patients (68%) had a favorable result; that is, a total collapse, or a marked reduction, of the syrinx. Seven patients (28%) were stable in terms of syrinx size. However, the syrinx enlarged in one patient who had undergone excision of the outer dura. Twenty-four patients achieved a widened cisterna magna with ascent of the cerebellar tonsils into the posterior fossa and acquisition of a more rounded shape. Postoperative complications included a transient headache and vomiting in three patients and transient motor weakness in one patient. Two patients developed a superficial wound infection. This study shows that arachnoid-preserving posterior fossa decompression is a safe and effective treatment for patients with CMI with associated syringomyelia.