We examined a deletion/insertion promoter polymorphism of the serotonin transporter gene, which confers an approximately 40% reduction in expression of the protein, in 196 subjects with late onset Alzheimer's disease (AD) and 271 controls. The frequency of the 484 bp low activity allele was elevated in the subjects with AD (p = 0.004), and an excess of the low activity genotype (30%) was also found in comparison with the controls (20%) (chi 2 = 7.16; p = 0.03). This association was unrelated to the age of the subjects or controls, or to epsilon 4 alleles of the ApoE gene. The odds ratio for the effect of the homozygous low activity genotype was 1.7 (95% CI 1.08-2.67), with a population attributable risk of 33% (95% CI 5-54%). These findings indicate that the low activity allele of the serotonin transporter is a risk factor for late onset AD.