Complex immunochemical and immunocytochemical investigation of postmortem frontal cortex (area 10) of normal controls and Alzheimer disease (AD) patients revealed a decrease in immunoreactive creatine kinase BB (CK-BB) content in AD. This decrease was not the result of age, postmortem delay, psychotropic therapy or astrocytic destruction. In some AD cases CK-BB was found in neurofibrillary tangles. The possible reasons for these findings are discussed. The decrease of CK-BB level may be important in the pathogenesis of AD because of the consequent energy deficit.