Macrophage-colony stimulating factor (M-CSF) and, less frequently, IL-1β and IL-6 were detected in the cerebrospinal fluid (SF) from Guillain-Barre syndrome (GBS) patients. IL-1α, IL-2, IL-10, TNFα, and IFNγ were not found. Detectable cytokine levels were not observed in chronic inflammatory demyelinating polyneuropathy (CIDP) SF nor in any of the sera studied. These findings suggest a prominent intrathecal activation of cells of the monocyte/macrophage lineage (M ) in GBS, and further support the hypothesis of a crucial role for M in GBS immunopathology.