Cystatin C is an amyloidogenic protein that colocalizes with beta-amyloid (Abeta) within arteriolar walls in Alzheimer disease (AD) brains. Recently, a coding polymorphism in the cystatin C gene (CST3) has been claimed to confer risk for the development of late-onset AD. In the present work we have tested the frequencies of CST3-A and CST3-G alleles and used chi-square and logistic regression analyses to assess the association among the CST3 polymorphism, apolipoprotein E4 (APOE4), and AD in a series of 159 AD patients and 155 controls. The CST3-A allele was seen to be an accumulation risk factor for early-onset AD. Furthermore, a synergistic association among the CST3-A allele, APOE4 and AD was found in AD patients whose ages were between 60 and 74 years.