OBJECTIVE. Astrocytomas are classified as either fibrillary or pilocytic on the basis of their histologic appearance. The imaging features of the fibrillary astrocytoma correlate closely with the tumor's clinical aggressiveness and are, therefore, useful in predicting prognosis. Correlation between the imaging features and the clinical aggressiveness of the pilocytic astrocytoma, however, is not well established. Accordingly, we compared the initial MR and CT appearances of the lesion with tumor aggressiveness as seen clinically to determine if a correlation exists. MATERIALS AND METHODS. We retrospectively evaluated the initial MR images or CT scans of 32 consecutive patients who had a histologic diagnosis of pilocytic astrocytoma. The lesions were evaluated with regard to location, size, calcification, morphology, and degree of contrast enhancement. These initial imaging features were correlated with the aggressiveness of the tumor as seen clinically. Tumors were classified as aggressive or nonaggressive on the basis of their clinical manifestations. Patients with clinically aggressive lesions had progressive symptoms and radiologic evidence of tumor progression or recurrence within an unusually short period. Patients with clinically nonaggressive lesions had a more indolent course, either improving or remaining stable, on both clinical and radiologic evaluations. In 12 patients, the tumor was classified as aggressive clinically, either progressing or recurring within a median time of 7.5 months (range, 2.5-118 months) from the initial diagnosis. The remaining 20 patients had a clinically nonaggressive course. RESULTS. In our series of patients, lesion size and location were not significantly different between the nonaggressive and aggressive tumors, as noted clinically. Furthermore, the aggressive and nonaggressive tumors were similar with regard to the presence or absence of calcium. Most tumors in both groups showed either moderate or marked enhancement and were multilobular. CONCLUSION. The initial CT and MR features of pilocytic astrocytoma are unreliable for predicting which lesions will behave in a more aggressive manner clinically and have a poor prognosis.