• The association of dementia of the Alzheimer type (DAT) with parental age at the birth of patients who develop DAT was investigated in a community-based study in Japan. Both maternal and paternal ages of patients who have developed DAT are significantly higher than those in controls and multi-infarct dementia. This study suggests that advanced age may become a cause of chromosome abnormality, and advanced parental age at subjects' birth may be a possible risk factor in DAT.