Axonal injury and glial activation are an early neuropathologic event in adults with multiple sclerosis. To investigate whether markers of axonal injury and glial activation are already elevated in the cerebrospinal fluid of children with multiple sclerosis, we studied the cerebrospinal fluid of 25 children with multiple sclerosis and 67 controls for the presence of tau, phospho-tau, and S-100B proteins using specific enzyme-linked immunoabsorbent assays. In general, tau, phospho-tau, and S-100B protein levels did not differ significantly between groups. However, in a subgroup of nine children with multiple sclerosis, all of whom had prominent clinical symptoms at the time of lumbar puncture and radiologic disease activity, tau protein levels were significantly elevated when compared with other controls. These data indicate that axonal injury is not restricted to adult multiple sclerosis but can already occur in children with multiple sclerosis.