In a study of 209 patients who had serum drawn for assay of creatine phosphokinase (CPK) isoenzymes, an attempt was made to correlate elevation of CPK brain fraction (BB) with occurrence of brain injury and to evaluate its sensitivity as an indicator of the presence of brain damage. Thirty-three patients had suffered serious head trauma. However, of the ten patients in coma, only three had elevated serum CPK-BB levels. Twenty-four of the other 176 patients who had no history and head injury also had positive BB fractions. Serum CPK-BB level was therefore an unreliable indicator of the extent of brain damage.