Background and Purpose-Homocysteine is an endothelial toxin and elevated levels have been associated with stroke risk.Stroke, particularly the small vessel disease (SVD) subtype, is increased in U.S. and UK black populations. In white populations elevated homocysteine has been associated with SVD, especially confluent leukoaraiosis, and may be acting through endothelial dysfunction. We determined the association between homocysteine and stroke subtypes, especially SVD, in a well-phenotyped UK cohort of black stroke patients compared to community controls. Methods-Homocysteine, vitamin B12, folate levels, and renal function were measured in 457 black stroke patients recruited consecutively through the prospective South London Ethnicity and Stroke Study and 179 black community controls. All patients were subtyped using modified TOAST criteria. Leukoaraiosis in SVD patients was graded according to severity, and patients were additionally categorized on the basis of presence or absence of confluent leukoaraiosis. Odds ratios (OR) and 95% confidence intervals (CI) were calculated. Results-The highest homocysteine levels were seen in SVD patients compared to controls (16.2 [11.6]