PURPOSE: To find characteristic MRI features of vermian medulloblastoma. MATERIALS AND METHODS: MRI studiesand medical records were retrospectively reviewed for 12 patients with surgically proven midline medulloblastoma. The assessment concerned appearance of the mass in relation to surrounding structures ; MR signal intensity ; the enhancement pattern ; the mass's location and size ; presence of a cystic/necrotic area, calcification, or vascular void ; extension through the foramen Luschka ; degree of hydrocephalus ; and presence of tonsillarherniation. RESULTS: The midline medulloblastoma commonly showed roundish moon-surface appearance, especially onthe axial T2-weighted images. All tumors showed heterogeneous signal intensities mainly due to intratumoralcystic/necrotic or hemorrhagic changes. The tumors were commonly located at mid- and/or inferior vermis. Occasionally the tumors extended through the foramen Luschka, and caused obstructive hydrocephalus of moderate tosevere degree. Post-con-trast study showed heterogeneous, dense contrast enhancement in the majority of patients. CONCLUSION: The MR finding of the moon-surface appearance formed by both the mass and the intratumoralcystic/necrotic change as seen on axial T2-weighted images could be helpful in the diagnosis of vermianmedulloblastoma.