The aim of the study was to investigate whether the regional distribution of white matter hyperintensities (WMH) observed by magnetic resonance imaging differed between vascular dementia and patients with late onset Alzheimer's disease. Another aim was to investigate the relations between the occurrence and degree of WMH and clinical and laboratory data as well as measures of cognitive decline. White matter hyperintensities were assessed with a low field magnetic resonance imager on 23 subjects with probable Alzheimer's disease, 25 with possible Alzheimer's disease and 31 subjects with vascular dementia. The degree and regional distribution of the WMH (expressed as relative volumes) were calculated and compared in the three groups. The relation between cognitive impairment and the degree of the WMH was also studied. The regional distribution of the WMH differed between the groups with significantly more changes in the posterior part of the brain (p < .0001) as well as in the right hemisphere (p < .0005) in the vascular demented as compared to the patients with Alzheimer's disease. No significant correlations between cognitive impairment and the degree of the WMH were found in any of the groups. The total volume of the WMH as well as the regional distribution of these changes differed significantly between vascular dementia and Alzheimer's disease. White matter hyperintensities seem not to be related to the degree of global cognitive decline in dementia and whether it plays a causative role in the development of dementia symptoms needs to be more thoroughly investigated.