In the last thirty years much progress has been made in the treatment of brain tumors in children, thanks to modern diagnostic imaging techniques, together with neurosurgery and radiotherapy, for the diagnosis and follow-up of these lesions. MRI and paramagnetic contrast agents have revolutionized the imaging of intracranial conditions, especially in the posterior cranial fossa. The superiority of MRI over CT is well known and reported in the literature, but the adequacy of the two methods has rarely been compared. Therefore, this study was aimed at assessing and quantitating the accuracy of MRI and CT relative to two essential parameters: preoperative "histologic" diagnosis and local and distant intracranial tumor extent. In 52 selected patients the final histologic diagnosis was cerebellar medulloblastoma in 20 cases, cerebellar astrocytoma in 12 cases, brain stem glioma in 12 cases and ependymoma in 8 cases. CT allowed the correct diagnosis to be made in 25 patients (48%), with questionable findings in 21 patients (40%) and 6 misdiagnoses (12%). MRI allowed the correct diagnosis to be made in 43 patients (83%), with questionable findings in 9 patients (17%) and no misdiagnoses. MRI depicted local and distant tumor spread in 34 patients while CT showed it in 22 patients only. The analysis of the results confirmed the superiority of MRI over CT, relative to the parameters considered, i.e. histology and the assessment of tumor spread. In the clinical suspicion of brain tumors of the posterior cranial fossa in children, MRI--if available--should be the examination of choice if the patient's clinical status does not prevent its use. CT should be used only in emergency cases (acute intracranial hypertension) to identify hydrocephalus and intratumoral hemorrhages. Unenhanced CT also yields useful pieces of information for lesion "histology" and may be used to integrate MR findings in selected cases.