Due to the increasing incidence of Alzheimer's disease (AD), many studies have aimed to improve its diagnosis. Particular attention has been focused on measuring volumes of brain structures. Only few studies have investigated whether the cerebellar volume changes with the stage of dementia. It is controversial whether the serum apolipoprotein E (ApoE) level is an appropriate AD marker. This study was designed to clarify the significance of both cerebellar volume measurements and ApoE level measurements as markers of neurodegenerative changes. This study included 55 subjects with AD, 30 subjects with mild cognitive impairments (MCI), and a control group with 30 subjects. We measured the brain, cerebellum, and brain stem volumes with magnetic resonance imaging (MRI). We determined serum ApoE levels, APOE genotypes, and neuropsychological test scores. In the control group, we found that ApoE levels were significantly higher for subjects with the APOE 2/3 genotype than those with the 4/4 genotype. This finding may indicate that ApoE plays a protective role against AD development in subjects with the APOE 2/3 genotype. ApoE levels were not significantly different in patients with AD and MCI. No correlations were found between serum ApoE levels and Mini-Mental State Examination (MMSE) scores or the volumes of brain structures. This study could not confirm the appropriateness of the cerebellum volume as an early AD marker. Correlations were found between cerebellar volume, brain volume, and the MMSE scores.