Thirty-seven cases of juvenile pilocytic astrocytoma were reviewed retrospectively to determine their CT and MR characteristics. All cases occurred in pediatric patients, except for one in a young adult. There was a propensity for tumors to be located around the third and fourth ventricles. On CT the tumors were all sharply demarcated and smoothly marginated and rarely had associated edema. The lesions tended to be round or oval. The tumor matrix was most often hypo- or isodense with marked enhancement. Cyst formation, either micro- or macrocystic or combined, was frequently observed, and tumor calcification occurred occasionally. On MR the tumors appeared hypo- or isointense on T1-weighted images and hyperintense on T2-weighted images. The radiologic appearances of juvenile pilocytic astrocytomas are quite characteristic. By using age of presentation, typical location, configuration, and enhancement patterns, the presurgical diagnosis of juvenile pilocytic astrocytoma can be made with a high index of confidence.