The central nervous system of nearly every HIV-positive patient becomes affected by the AIDS virus itself or by one of the associated diseases during the course of the illness. Early diagnosis of lesions which demand therapeutic consequences is of the most importance concerning prolongation of life and improvement in its quality. In spite of the frequent underestimation of cerebral involvement by imaging methods and their unspecific findings they are often the only diagnostic means which permit-timely diagnosis and, at least in some diseases, therapeutic monitoring. Indications for cranial computed tomography (CCT) or magnetic resonance tomography (MRT) are already present with mild or transient neurological or psychiatric symptoms or the extracerebral manifestation of neurotropic organisms or tumours which metastasize to the brain, even in patients without subjective complaints.