Alzheimer disease (AD) is the most common type of dementia. In this review article, we summarized the current findings concerning the prevalence and incidence of AD in Japan and other countries; these findings were reported in the last 10 years. The prevalence of dementia among individuals aged more than 65 and more than 75 years was 2.4%-8.4% and 15.8%, respectively; this was determined according to the DSM-IV or DSM-III-R criteria. Further, the worldwide ratio of AD and vascular dementia (VaD) was 1.8-4.8, which was calculated according to the NINCDS-ADRDA and NINDS-AIREN criteria. In Japan, the prevalence of dementia among individuals aged more than 65 years was 3.8%-11.0%; this value was calculated according to the DSM-IV or DSM-III-R criteria. Further, the AD/VaD ratio among these individuals was 2.1-4.1, which was calculated according to the NINCDS-ADRDA and NINDS-AIREN. The prevalence of dementia and the AD/VaD ratios vary widely among the previous reports. The reasons for this are variations in the diagnostic criteria, epidemiological methods, demographics caused by the aging population, and mortality rate. It is necessary to establish a standard international epidemiological method worldwide. In addition, the diagnostic criteria of other types of dementias such as dementia with Lewy bodies and idiopathic normal-pressure hydrocephalus should be considered.