Background and Purpose-Individuals of African Caribbean descent who live in the United Kingdom have an increased risk of stroke. The reasons for this are not fully understood, but differences in genetic predispositions or other novel stroke risk factors could play a role. US blacks have been reported to have increased common carotid artery wall thickness, or intima-media thickness (IMT), measured by ultrasound. We measured carotid IMT in UK African Caribbeans compared with UK whites and determined whether different distributions of polymorphisms in potential candidate vascular genes or differences in measures of chronic inflammation or infection could account for any difference. Methods-In a population study, common carotid artery IMT was measured in 202 white men and 89 African Caribbean men. The distribution of polymorphisms in ACE, paraoxonase 1, paraoxonase 2, and methylenetetrahydrofolate reductase genes was determined. Serum C-reactive protein and Helicobacter pylori seropositivity were determined. Results-Carotid IMT was increased in African Caribbeans even after controlling for cardiovascular risk factors, including homocysteine and social class: ␤ϭ0.113, 95% CI 0.036 to 0.189, Pϭ0.004. There was a significant interaction with smoking and mean IMT (Pϭ0.022), and the difference in both measures of IMT between ethnic groups was largely limited to individuals who had never smoked. There were significant ethnic differences in the distributions of 3 of the 4 candidate genes studied (ACE, paraoxonase 1, and methylenetetrahydrofolate reductase). H pylori seropositivity was increased in African Caribbeans (78.7% versus 53% in UK whites). However, neither the genetic polymorphisms nor H pylori seropositivity was related to IMT, and ethnic differences in their distribution did not account for the increased IMT seen in African Caribbeans. Conclusions-Carotid IMT is increased in UK African Caribbeans even after controlling for conventional risk factors.There are highly significant ethnic differences in the distribution of many potential cerebrovascular candidate genes. Although those we examined did not explain the ethnic differences in IMT, other genetic predispositions or environmental exposures could account for these differences.