BACKGROUND/AIMS: We performed a morphometric analysis of Chiari I malformations to look for predictors of cervical syrinx formation. METHODS: Eighteen patients with Chiari I malformation and associated cervical syrinx and 16 patients with Chiari I malformation without associated cervical syrinx were included in the study. Chiari I size was obtained from the radiology report; foramen magnum diameter, cerebellar volume, posterior fossa volume and intracranial volume were calculated using OsiriX software, and average measurements were compared between the two groups. RESULTS AND CONCLUSION: Patients with Chiari I with syrinx had an average tonsillar descent of 13.03 ± 5.31 mm compared to 9.25 ± 3.31 mm in the Chiari I without syrinx group (p < 0.05). Patients with Chiari I and syrinx also showed increased cerebellar crowding with a higher cerebellar volume to posterior fossa volume ratio; however, this difference was not significant (0.83 vs. 0.81; p = 0.1872). No difference between groups was found in posterior fossa volume, intracranial volume and foramen magnum diameter. Therefore, only Chiari I size based on the extent of tonsillar herniation was found to be a determinant of cervical syrinx formation.