Apolipoprotein E (APOE) gene on chromosome 19q13.2 is encoded by three common alleles designated as epsilon2, epsilon3 and epsilon4. In Alzheimer's disease (AD) the epsilon4 allele is over-represented and is considered to be a major genetic risk factor. Several methods have been developed to determine APOE genotypes. Among them, polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) appears to be highly reliable. In this study, we improved the nonisotopic PCR-SSCP method for determining APOE genotypes in 42 cases of AD patients, 40 cases of non-AD dementia patients, and 49 cases of age-matched controls. DNA from the target sequence on APOE was amplified by PCR from peripheral blood genomic DNA. PCR products were electrophoresed in a non-denaturing polyacrylamide gel and visualized by silver staining. We found that the epsilon4 allele had a significantly high frequency of occurrence in AD patients (33.3%) compared with age-matched controls (13.3%) (chi(2) = 10.43, p = 0.001) and non-AD dementia (10%) (chi(2) = 13.02, p<0.001) whereas the epsilon3 allele was of high frequency in non-AD dementia (90%) compared with age-matched controls (85.7%) and AD patients (66.7%). APOE epsilon4 homozygotes were found only in AD groups. On the other hand, the epsilon2 allele was found only in an age-matched control. This study confirmed that the APOE psilon4 allele is a risk factor in Thai AD subjects and that the PCR-SSCP method is a rapid and useful means of detecting the APOE genotype in AD.