Background Stroke risk is higher in black ethnic groups compared with white. Although risk factors for stroke are known to differ between these populations, few population studies have reported on the risk of aetiological stroke subtypes in black ethnic populations.

Methods Ethnic group differences in incidence of first ever ischaemic stroke by aetiological subtype were investigated with the South London Stroke Register (SLSR). The SLSR is a population based stroke register covering a multi-ethnic inner city population of 271 871 inhabitants comprising 63% white, 28% black and 9% other ethnic group. A modified pathophysiological Trial of ORG 10172 in Acute Stroke Treatment (TOAST) classification of stroke was used to estimate patterns of aetiological subtype and stroke was subtyped into large artery atherosclerosis (LAA), cardioembolism (CE), small vessel occlusion (SVO), other aetiology (OTH), undetermined (UND) and multiple possible or concurrent aetiologies (CONC).

Results Between September 1999 and August 2006, 1181 patients with first ever ischaemic stroke were included in the study. Mean age was 71.4 years, 51% were female and 71% were white patients, 20% were black patients, 6% were other and 3% were of unknown ethnic group. The distribution of the aetiological subtypes was as follows: LAA, 109 (9.3%); CE, 325 (27.8%); SVO, 316 (27.0%); OTH, 40 (3.4%); UND, 283 (24.2%) and CONC, 96 (8.2%). The annual age adjusted incidence rate per 100 000 was for total ischaemic stroke 101.2 (95% CI 82.4 to 122.9) in men and 75.1 (95% CI 59.1 to 94.1) in women; for LAA 10.4 (95% CI 5.1 to 18.9) in men and 6.8 (95% CI 2.7 to 14.2) in women; for CE 23.0 (95% CI 14.6 to 34.5) in men and 21.5 (95% CI 13.4 to 32.8) in women; and for SVO 30.3 (95% CI 20.5 to 43.2) in men and 20.3 (95% CI 12.5 to 31.3) in women. The overall incidence rate ratio (IRR) for black patients was 1.25 (1.07 to 1.46), for black Caribbean (BC) patients 1.31 (1.09 to 1.58), for black African (BA) patients 1.22 (0.93 to 1.61) and for other ethnic groups 1.24 (0.96 to 1.61). IRRs for black ethnic groups as well as for BA and BC were significantly higher for SVO in both sexes, for OTH in black patients for females and in BA for males and females compared with the white ethnic group; IRRs for other ethnic groups compared with white patients were higher for SVO in females and for UND in males.

Interpretation Independent important differences in risk of stroke between different ethnic populations strengthen the evidence base for studying genetic susceptibility and environmental influences in ethnic groups separately.