To study white matter changes in the frontal lobes and to investigate the histopathological basis for the dementia in progressive subcortical vascular encephalopathy (PSVE), the frontal white matter was examined by electron microscopy in seven cases with PSVE, and compared with that in a control group and in cases with senile dementia of Alzheimer's type (SDAT). The number of nerve fibres per unit selected area of the white matter was significantly less in PSVE compared with that in the control group or in SDAT. Nerve fibres in PSVE had a tendency to have thinner myelin sheaths than in the control group or in SDAT, but the difference was not significant. The pallor of the frontal white matter in PSVE is mainly based on the loss of nerve fibres, and may be in part based on the thin myelin sheaths. The dementia in PSVE is probably related to the loss of nerve fibres in the cerebral white matter.