Hippocampal size on magnetic resonance imaging was compared between normal subjects with the apolipoprotein E (apo E) ε4 allele (ε44, ε43, and ε42) and those without the ε4 allele (ε33, ε32, and ε22) in the age range of 39-80 years. The Mini-Mental State Examination (MMSE) scores did not differ between the two groups. The right hippocampal area and its ratio to hemisphere area and intracranial cavity area were significantly smaller in ε4 carriers than non-carriers, whereas hemisphere area did not differ between the two groups. These results suggest that as early as their forties, apo E ε4 allele carriers have a markedly smaller right hippocampus with no apparent cognitive impairment, which may have some significance in the high prevalence of the ε4 allele in Alzheimer's disease as well as other conditions that cause dementia.