This chapter talks about an 85-year-old man who was enrolled in a longitudinal study of healthy aging and Alzheimer disease (AD) at the Washington University Alzheimer Disease Research Center (ADRC). He was diagnosed with dementia of the Alzheimer's type (DAT) and died shortly after his 91st birthday. A psychometric battery was performed within 2 weeks of each annual clinical assessment. The detection of dementia at the ADRC is based on observations from a collateral source of decline from an individual's previously attained level of cognitive function that is sufficient to interfere with that individual's customary activities. This case illustrates that the accurate diagnosis of DAT can be made in individuals who meet criteria for mild cognitive impairment (MCI). The DAT diagnosis was based on informant observations that the individual had declined from previously attained levels of cognitive function, rather than comparing his cognitive performance to normative values.