The cytokine tumor necrosis factor (TNF) was assayed in the sera (n = 31) and cerebrospinal fluid (n = 26) of children with acquired immunodeficiency syndrome, using a competitive radioimmunoassay. Elevated serum levels of TNF were found in 15 (79%) of 19 patients with progressive encephalopathy (PE), compared with 1 (8%) of 12 patients without neurologic involvement. There was a significant association of PE with elevated serum TNF levels. Conversely, of 16 patients with elevated serum TNF levels, 15 (94%) were found to have PE, and of 8 patients with serum TNF levels greater than 100 pg/ml, all 8 (100%) had PE. No association was found between cerebrospinal fluid levels of TNF and PE. Neither serum nor cerebrospinal fluid TNF levels correlated with the degree of cachexia. These data suggest that circulating TNF may be responsible for the myelin damage that occurs in human immunodeficiency virus type 1-associated PE.