MR detected abnormality in all 115 pediatric patients who subsequently had pathologically proven posterior fossa tumors. In 114, the initial magnetic resonance (MR) diagnosis was that of brain tumor. In 1, with less than 1-cm2 area of gadolinium enhancement, the significance of the initial finding was uncertain. Common posterior fossa tumor subgroups (brainstem gliomas, cerebellar astrocytomas, primitive neuroectodermal tumors, and ependymomas) have relatively consistent presentations on imaging studies. However, less common tumors mimic the MR appearance of more common ones, while common tumors may also have atypical appearances.