HIV-induced neurological disease is postulated to be caused by indirect mechanisms. Tumor necrosis factor (TNF)alpha is increased in the brains in human immunodeficiency virus (HIV)-associated dementia and in the spinal cord in vacuolar myelopathy and may play a pathogenetic role in these diseases. Microglia, astrocytes and infiltrating macrophages can be induced to produce TNF alpha and each has been identified as a source of TNF alpha in neurological disease. Reverse transcriptase synthesis of cDNA and polymerase chain reaction amplification of the cDNA was combined with immunocytochemistry to identify the cellular source of TNF alpha in HIV-induced neurological disease. Cells positive for TNF alpha mRNA were more abundant in white matter than gray matter of the brain from demented individuals. TNF alpha mRNA-positive cells in brains and spinal cords were almost exclusively macrophage-lineage cells. Only rare TNF alpha mRNA-positive cells were astrocytes. We conclude that macrophage-lineage cells are the primary source of elevated central nervous system TNF alpha mRNA in providing further evidence that macrophage activation is an important element in the pathogenesis of HIV-associated neurological disease.