Ventricular dilation affects 7% to 10% of patients with Chiari type I malformation (CIM), but the choice of surgical treatment is controversial. To study the surgical approaches for treating CIM with ventricular dilation and to evaluate the efficacy of posterior fossa decompression (PFD), clinical and imaging data of 38 adult patients who received surgical correction performed at the authors' department from 2004 to 2011 were reviewed. Ventricular dilation was defined as Evans' index > 0.30. Preoperative fundus examinations were done on all patients and no papilledema was found. Surgical procedures included a sub-occipital decompression and a C1 laminectomy, followed by a duraplasty with an autologous graft. Evans' index was measured before and after surgery respectively, and a paired samples t-test was performed to examine the difference. Factors predicting outcomes were investigated using logistic regression analysis. Follow-up was done to all patients with an average duration of 43 months. All postoperative magnetic resonance (MR) images showed a relief of cervicomedullary compression and recreation of the cisterna magna. Symptoms improved in 33 patients (86.8%), remained stable in 5 (13.2%), and no patient deteriorated. No significant change in ventricular size was observed after surgery (P = 0.257). Regression analysis showed duration of illness had a significant effect on clinical outcome (P = 0.034, OR = 12.5, 95% CI: 1.214, 128.661). Our study suggests that the intracranial pressure (ICP) of patients with CIM and ventricular dilation is usually normal. PFD with duraplasty is an effective and safe treatment for CIM with ventricular dilation. Treatment of ventricular dilation is unnecessary before PFD as long as there is no persistent headache, vomiting, and papilledema.