Neurofibromatosis 1 (NF-1) (von Rekhlinghausen disease) and neurofibromatosis 2 (NF-2) (bilateral acoustic neurofibromatosis) have been recently recognized to be distinct disorders through genetic linkages. The authors compared the cranial magnetic resonance (MR) images of 53 patients with NF-1 and 11 with NF-2. In the NF-1 group, MR imaging revealed 19 patients with optic gliomas and eight with parenchymal gliomas. In 32 patients, foci of prolonged T2, similar to those reported previously as hamartomas, were identified in the cerebellar peduncles, globus pallidus, midbrain, and other locations. The frequency of these foci was related to both age and the presence of optic gliomas. In the NF-2 group, MR imaging revealed eight patients with cranial nerve schwannomas and six with meningiomas (in addition to acoustic schwannomas in all 11). These findings demonstrate that NF-1 and NF-2 are different diseases requiring different imaging protocols. NF-1 seems to be associated with tumors of astrocytes and neurons and NF-2 with tumors of meninges and Schwann cells.