We investigated the status of the apolipoprotein E allele in 538 participants in the incidence phase of the ongoing Shanghai Survey of Dementia, including 103 demented subjects, 72 with mild cognitive impairment and 363 cognitively normal. The apo E ε4 allele was present in 10.2% of control subjects and the allelic frequency did not change between ages 60 to 96 years. The apo E ε4 allelic frequency was increased both in those with Alzheimer's disease (AD) (25.4%) and those with vascular dementia (VaD) (22.2%), but not in those with other dementing illnesses or the cognitively impaired. All of the subjects homozygous for apo E ε4 were demented, three were diagnosed as having AD, and three met NINDS/AIREN criteria for VaD. The increased apo E ε4 allelic frequency in clinically diagnosed VaD patients suggests that some of the infarcts are secondary to congophilic angiopathy. The adjusted odds ratio of developing AD in this community-derived study for persons with at least one apo E ε4 allele was 4.1 (95% CI: 2.2, 7.7). Thus, the apo E ε4 risk of developing AD in this Chinese cohort is similar to that in western community studies.