Opportunistic infections of the central nervous system, frequent in AIDS, present essentially in 4 clinical pictures: brain mass lesion(s), meningitis, diffuse encephalitis or prolonged fever. Toxoplasmosis is by far the most frequent cause of brain mass lesion. Therefore, for patients presenting with a brain mass lesion it is recommended to start presumptive antitoxoplasmic therapy with the combination of pyrimethamine and sulfadiazine. Other etiologies (lymphoma and progressive multifocal leukoencephalopathy) should be considered only after failure of this treatment. The other infectious processes are much more uncommon (cryptococcoma, tuberculoma, Candida, Nocardia abscesses). In case of meningitis, cryptococcosis must be searched for, in addition to classic causes of meningitis. Diffuse febrile encephalitis should lead to consider presumptive antitoxoplasmic therapy. Finally, prolonged fever can be the only expression of cerebral toxoplasmosis or cryptococcal meningitis.