Technical Abstract: Endothelial nitric oxide synthase (NOS3) activity and cigarette smoking significantly influence endothelial function. We sought to determine whether cigarette smoking modified the association between NOS3 polymorphisms and risk of coronary heart disease or stroke. All 1085 incident coronary heart disease cases, all 300 incident ischemic stroke cases, and 1065 reference individuals from the Atherosclerosis Risk in Communities study were genotyped for the T-786C and E298D polymorphisms in NOS3. Using a case-cohort design, associations between genotype/haplotype and disease risk were evaluated by multivariable proportional hazards regression. Multiplicative scale interaction testing evaluated the influence of cigarette smoking history at baseline on these associations. In Caucasians, association between E298D genotype and risk of coronary heart disease was significantly modified by current smoking status (interaction P=0.013), with the highest risk observed in smokers carrying the variant D298 allele relative to nonsmokers carrying two E298 alleles (adjusted hazard rate ratio 2.07,95% confidence interval l1.39–3.07). In African-Americans, association between T-786C genotype and risk of ischemic stroke was significantly modified by pack-year smoking history (interaction P=0.037), with the highest risk observed in greater then or equal 20 pack-year smokers carrying the variant C-786 allele relative to smaller then 20 pack-year smokers carrying two T-786 alleles (adjusted hazard rate ratio 4.03,95% confidence interval 1.54–10.6).