PURPOSE: To determine by analysis of MR and CT findings the points of differentiation among 4th ventricular tumors, especially the change of shape of the 4th ventricle caused by the site at which 4th ventricular tumors originate. MATERIALS AND METHODS: The authors retrospectively analyzed and compared the CT(n=5) and MRI(n=12) findings of 13 pathologically proven 4th ventricular tumors comprising six medulloblastomas three ependymomas (4 cases) and three choroid plexus papillomas. RESULTS: On axial MRI medulloblastomas showed anterior and anterolateral CSF-clefts between the tumor mass and the 4th ventricular wall in one and five cases, respectivaly ; on sagittal MRI, anterior beaking of the upper 4th ventricle was seen. Two ependymomas showed posterolateral CSF-cleft on axial MRI and posterior beaking of the upper 4th ventricle on sagittal MRI. Two ependymomas and all choroid plexus papillomas showed anterior, posterior and lateral CSF-clefts on axial MRI, and anterior and posterior beakings of upper 4th ventricle on sagittal MRI. On Gd-DTPA enhanced T1WI, all medulloblastomas and ependymomas showed inhomogeneous enhancement, and all choroid plexus papillomas showed homogeneous enhancement. On CT, tow choroid plexus papillomas showed dense calcifications. CONCLUSION: The differential diagnosis of 4th ventricular tumors can be preoperatively suggested by analysis of findings such as a CSF-cleft between the tumor mass and the 4th ventricular wall on axial MR and CT images, the shape of the upper 4th ventricle on sagittal MRI, contrast enhancement pattern, necrosis and cyst, and CSF seeding.