We measured levels of alpha-tumor necrosis factor (alpha-TNF) in cerebrospinal fluid and serum samples from 50 drug-free patients with multiple sclerosis, 25 patients with other neurological diseases, 27 patients with non-neurological diseases, and 10 normal subjects. The most elevated levels of alpha-TNF were found in patients with inflammatory or autoimmune diseases. Comparable serum levels of alpha-TNF were detected in normal control subjects, patients with multiple sclerosis, and patients with degenerative neurological diseases. In patients with multiple sclerosis, alpha-TNF levels were also unrelated to time elapsed between the occurrence of clinical exacerbation and the time of sample collection. Only 3 patients with chronic progressive multiple sclerosis had detectable alpha-TNF in the cerebrospinal fluid. Our data do not support a role for elevated levels of circulating alpha-TNF in the maintenance of the disease. However, we cannot rule out the possibility that a transient elevation of alpha-TNF triggers the cellular events leading to demyelination in multiple sclerosis.