Although associations between ABO blood type groups and personality traits have been treated as pseudoscience, we observed a significant association between ABO blood group genotype and personality traits in a large number of healthy Japanese subjects. However, these results should be regarded as preliminary and should be interpreted with caution because it is possible that the association between ABO blood group genotype and the Persistence trait is relatively weak. Further studies of this relationship between ABO blood type and personality traits could promote the understanding of human behavior and advance efforts to assess future risks of illness. ABO Blood Type and Personality Traits in Healthy Japanese Subjects