To determine the relationship between apolipoprotein E (APOE) epsilon4 and sporadic Alzheimer's disease (SAD) in the Mongolian population in China, we examined 106 Mongolian SAD patients in China and 100 Mongolian healthy controls. All subjects were genotyped for APOE. The odds ratio (OR) for developing SAD was significantly increased in carriers of the APOE epsilon4 allele compared with APOE epsilon4 non-carriers (chi(2)=5.59, df=1, P=0.018, OR=1.84, 95% CI 1.02-3.31). Our findings suggested that APOE epsilon4 is a high-risk factor for AD in the Mongolian population in China. The present study is the first to establish the relationship between APOE epsilon4 and SAD in the Mongolian population.