An epidemiological study on senile dementia was carried out among 1,390 people over 60 years of age in Haidian district, Beijing. All the elderlies were from the communities. Senile dementia was diagnosed according to MMSE. AD was diagnosed according to DSM-III-R and MID by modified Hachinski score. The morbidity of senile dementia was 2.33% (AD was 0.37% and MID was 0.96%). The morbidity of AD was similar to those in other parts of China, but the morbidity of MID was lower. People with older age and lower education seemed to be more susceptible to AD and MID, whereas females to AD.