Early events of HIV infection of the CNS are not yet clear. HIV infection in most recent cases, generally shows a prolonged interval between diagnosis and death. HIV infection, months to years before the patient's death, may or may not result in early neurologic symptoms. AIDS patients with spinal cord symptoms often show a sudden onset of long tract signs and a temporally related altered mental status indicating the appearance of both myelitis and encephalitis. Immunohistochemical localization of the HIV cell-surface receptor protein, CD4, and of HIV antigens in cerebral and lymph node venular endothelial cells suggests that a natural occurrence or induction of CD4 protein in some endothelial cells allows transmission of HIV from circulating infected white blood cells preferentially to certain tissues through endothelial cell infection. HIV immunolocalization is present in perivascular astrocytes, particularly in long white matter tracts, and on the surface of oligodendrocytes. HIV immunoreactivity is mostly in macrophages and multinucleated cells in a typical autopsy case, but this may be due to the clearing of HIV antigen from early sites of infection by the hematogenous cells. Not all immunoreactivity for HIV antigens is necessarily due to HIV gene products. Cross reacting epitopes, such as that of HIV envelope glycoprotein gp120 and neuroleukin, may be "seen" not only by antibodies on tissue sections, but by an AIDS patient's immune system, thus targeting a CNS antigen for immune-complex formation. Evidence for hypersensitivity disease in the CNS in AIDS includes the frequent findings of demyelination, perivenous chronic inflammation, chronic vasculitis, and perivenous hemorrhages. The white matter demyelination so frequently reported in all areas of the CNS in AIDS could be the result of a combination of factors that include direct HIV vasculitis, opportunistic infections, and hypersensitivity responses. The blood-brain barrier is breached when immune-related antigens interact on CNS vascular endothelial cells. Perhaps the CD4 antigen, which responds to interaction with antigen-presenting cells and enhances cellular immune activity, is induced or increased in the CNS in association with immune activity and in the presence of a leaky blood-brain barrier. Therefore, with or without HIV in the CNS, hypersensitivity disease, including demyelination, may be the result of long-standing activity of the immune system in AIDS patients.