Changes that may appear in the central nervous system in the course of AIDS either result directly from HIV infection or--as is the case with opportunistic infections and some neoplasms--develop as a secondary consequence of general immunodeficiency. Neuroimaging techniques may be most useful in the differential diagnosis of these lesions. Basic principles of HIV encephalitis and progressive multifocal leukoencephalopathy differentiation in MRI scans are discussed in the paper, and diagnostic possibilities of MR imaging in some other infections (tuberculosis, toxoplasmosis, and cryptococcosis) are outlined. Special attention is paid, on the one hand, to difficulties in the differentiation between toxoplasmosis and lymphoma, and on the other hand--to the growing diagnostic utility of MR-spectroscopy in this respect.