The serum levels of apolipoprotein B, C-II and the ratio of apolipoprotein B/A-I were found to be significantly higher in multiinfarct dementia than in dementia of the Alzheimer type. The high serum levels of apolipoprotein B, C-II and the high ratio of apolipoprotein B/A-I may cause multiinfarct dementia. The large difference between the groups suggests that the serum levels of apolipoprotein B, C-II and the ratio of apolipoprotein B/A-I may be useful in differential diagnosis of these two kinds of dementia.