We aim to investigate the pathological background of mild cognitive impairment (MCI). The most recent 545 cases from the Brain Bank for Aging Research (BBAR) were studied, with a mean age of 80.7 years and male : female ratio of 324 : 221. Cases with clinical dementia rating scale (CDR) 0.5 were retrieved as the best substitute of MCI. CDR was retrospectively determined from clinical charts. Pathological examinations followed the BBAR protocol (JNEN 2004). Post mortem assessment of CDR was possible for 486 cases, and was 0 in 201 cases, 0.5 in 57 cases and 1-3 in 228 cases. CDR 0.5 group was clinicopathologically classified into 33 cases with degenerative changes, nine cases with vascular changes, four cases with combined degenerative and vascular changes, two with hippocampal sclerosis, two with trauma, one with metabolic disease and six with unremarkable changes. The degenerative group was further subclassified into groups with pure and combined pathology. The former consisted of six cases each with Alzheimer change (AC), argyrophilic grain change (AGC) and neurofibrillary tangle predominant change (NFTC), three each with Lewy body disease change without parkinsonism (DLBC) or Parkinson's disease (PDMCI) and one case with progressive supranuclear palsy. The latter consisted of three cases with AC plus AGC, two with AGC plus NFTC and one each with AC plus DLBC, DLBC plus amyotrophic lateral sclerosis and AGC plus DLBC. The pathological backgrounds of patients of class CDR 0.5 were varied and not restricted to AC.