The phenotypes of apolipoprotein E (ApoE) in the plasma of patients with dementia of the Alzheimer type (DAT) and vascular dementia (VD) were determined by the isoelectric focusing method. The ApoE mRNA level in the skin fibroblasts was also determined by the Northern blot analysis. As compared with the control subjects, the frequency of the ApoE epsilon 4 allele was significantly higher in the DAT group as well as the VD group, but was not significantly different in the cerebrovascular disease without dementia (CVD) group. The skin fibroblast ApoE mRNA level in the DAT group and the VD group was significantly lower than that in the control group. These findings suggest that the phenotype of ApoE is associated with DAT and VD, and that the lower level of ApoE mRNA may play an important role in the development of DAT as well as VD.