Objective To examine the prevalence of atrial fibrillation (AF) and stroke risk by ethnic group in south and east London; to compare classification with CHA₂DS₂VASc and CHADS₂; to examine the appropriateness of anticoagulant treatment and historic trends in prescribing by gender, age, and ethnicity.

Design Cross-sectional study.

Setting Routine general practice records from south and east London.

Patients Patients aged 18 years or over with AF.

Main outcome measures Risk of stroke by CHA₂DS₂VASc and CHADS₂ score, and prescription of anticoagulant.

Results In 2011, we identified 6292 patients with AF, with an age adjusted prevalence of 0.63% (1.2% white, 0.4% black African/Caribbean and 0.2% South Asian). 93% of the AF population were at high risk of stroke with a CHA₂DS₂VASc score ≥1, of whom 54% were on warfarin. South Asian patients were at higher stroke risk than white patients (OR 1.67, 95% CI 1.02 to 2.73). Warfarin under-prescribing in people over 80 years of age persisted without improvement throughout 2008–2011. There were no clear differences in warfarin use by ethnic group.

Conclusions Despite a reduced prevalence of AF among South Asian patients, their risk of stroke is higher than for white patients or black African/Caribbean patients in association with diabetes, cardiovascular disease, and hypertension. Under-prescription of anticoagulation persists in all ethnic groups, a deficit most pronounced in the elderly. Use of the CHA₂DS₂VASc score would enhance optimal management in primary care.