To assess the role of alpha-tumor necrosis factor in the pathogenesis of central nervous system involvement during human immunodeficiency virus type 1 infection, we recorded clinical data and measured alpha-tumor necrosis factor levels in serum and cerebrospinal fluid samples from 45 patients infected with human immunodeficiency virus type 1, classified as group II/III (10), group IV A (5), group IV B (10), and group IV C-1 (20) of the Centers for Disease Control acquired immunodeficiency syndrome classification system and 42 controls. Alpha-tumor necrosis factor was above the limit of detection in only 3 of 15 sera and 3 of 15 cerebrospinal fluid samples from patients in group II/III and group IV A, whereas it was detected in 17 of 30 sera (p less than 0.05) and 22 of 30 cerebrospinal fluid (p less than 0.0002) samples from clinically more advanced patients (group IV B and group IV C-1). Alpha-tumor necrosis factor mean values were 21.5 pg/ml in sera and 50.0 pg/ml in cerebrospinal fluid from group IV B patients and 30.4 pg/ml in sera and 24 pg/ml in cerebrospinal fluid from group IV C-1 patients.(ABSTRACT TRUNCATED AT 250 WORDS)