Human leukocyte antigen (HLA), which is critically involved in immune response to foreign antigens and in autoimmunity, has been implicated in dementia and Parkinson's disease. Here we report on the correlations between the population frequencies of 127 HLA Class I and II alleles and the population prevalence of dementia and Parkinson's disease in 14 Continental Western European countries, extending previous work. We used these correlations to construct and compare HLA profiles for each disease. We found that (a) the HLA profiles of the two diseases were significantly correlated across both HLA Class I and Class II alleles, (b) negative ("protective") HLA-disease correlations did not differ significantly for either HLA class, but (c) positive ("susceptibility") HLA-disease correlations were significantly higher in dementia than in Parkinson's disease for both HLA classes of alleles. These findings indicate that (a) dementia and Parkinson's disease share immunogenetic HLA-related mechanisms, (b) HLA-related protective mechanisms (presumably against pathogens) do not differ between the two diseases, but (c) HLA-related susceptibility mechanisms (presumably underlying autoimmunity) are significantly stronger in dementia than in Parkinson's disease.