Serum levels of myelin basic protein (M.B.P.), a nervous-system-specific protein, were measured in 157 patients after head injury and related both to the type of brain damage and to the clinical outcome assessed three months after injury. Mean concentrations of M.B.P. in patients with severe intracerebral damage, with or without associated extracerebral haematoma, were significantly raised at the time of admission and remained high for two weeks after injury. In patients with extracerebral haematoma not associated with severe intracerebral damage mean M.B.P. values rose four to six days after injury and were significantly raised only in patients with poor eventual outcome. Mean serum-M.B.P. concentrations in patients with a good outcome after injury were similar to those in controls. In patients with a poor outcome the mean M.B.P. levels between two and six days after injury were significantly higher than in those with a good outcome. The assay of serum-M.B.P. may be valuable in assessment of severity of brain damage in patients after head injury and in prediction of outcome.