Brain-type creatine kinase (CK) isoenzyme (CK-BB) was found in the ventricular cerebrospinal fluid (CSF) and the serum in a series of 35 patients within 13 hours following severe head injury. There was a good correlation between total CK and CK-BB activities in CSF only. The values found in the CSF appear to be a quantitative index of brain dysfunction at admission, and did not correlate with intracranial pressure activity. High levels of CK-BB in the CSF correlated with a poor outcome, and thus offer a reliable criterion for the assessment of the severity of head injury.