MR tomography was performed on 23 children with tumors in the posterior fossa. In 14 cases, the MR tissue parameters were determined. Astrocytomas can be easily differentiated from medulloblastomas and ependymomas on the basis of their localization and signal intensities. Grading of astrocytomas and distinction between medulloblastomas and ependymomas was not possible. Because of high tissue contrast, multiplanar imaging and minimal artifacts, MR proved to be the method of choice for planning surgery and for the follow-up of tumours in the posterior fossa.