Human immunodeficiency virus (HIV) may be the cause of both primary and secondary brain diseases. In this review general features of HIV-associated neuropathology are discussed. Up to 90% of patients with AIDS have a variety of HIV-related brain diseases. Primary brain diseases including lymphocytic meningitis and HIV encephalitis are attributed directly to the effect of the virus on the brain. Secondary diseases including toxoplasmosis, cryptococcosis, primary leukoencephalopathies and lymphomas result from these patients' immunodeficiency status.