OBJECTIVES: We attempted to explore of the nature of very early or mild Alzheimer disease due to its crucial importance in understanding natural history, assessment of therapeutic interventions and accurate estimation of prevalence and incidence of Alzheimer disease. METHODS: We compared the scores of cognitive functions measured with Cognitive Impairment Diagnosing Instrument(CIDI) in questionable dementia of Alzheimer type(QDAT), mild dementia of Alzheimer type(bHDAT), and control. The three groups were controlled for age, sex, and education. RESULTS: Total score of the CIDI was not significantly different between QDAT and MIDAT. The subjects with QDAT and MIDAT scored lower in short-term memory, concentration/calculation, and abstract thinking than controls, while the QDAT and MIDAT subjects scored similarly in those subjects of the CIDI. CONCLUSION: These findings suggest that QDAT may be very early or mild Alzheimer dementia.