OBJECTIVE: To determine the prevalence of atrial fibrillation in England and Wales, and examine trends in its treatment with warfarin and aspirin between 1994 and 1998. DESIGN: Analysis of data from the general practice research database. SETTING: England and Wales. PATIENTS: 1.4 million patients registered with 211 general practices. MAIN OUTCOME MEASURES: Age and sex specific prevalence rates of atrial fibrillation; percentage of patients with atrial fibrillation treated with oral anticoagulants or aspirin. RESULTS: The prevalence of atrial fibrillation in 1998 was 12.1/1000 in men and 12.7/1000 in women. Prevalence increased from less than 1/1000 in under 35 year olds to over 100/1000 in those aged 85 years and over. There was a 22% increase in the age standardised prevalence of atrial fibrillation in men and a 14% increase in women between 1994 and 1998. The percentage of patients prescribed oral anticoagulants increased from 20% to 34% in men and from 17% to 25% in women. The percentage of men with atrial fibrillation prescribed aspirin increased from 26% to 36%, and the percentage of women increased from 24% to 36%. Applying the age and sex specific prevalence and treatment rates to the population gives an estimate of around 650 000 cases of atrial fibrillation in England and Wales. The greatest number of cases occurs in the 75-84 year old age group. CONCLUSIONS: The number of patients in the community with identified atrial fibrillation is increasing. There has also been a pronounced increase in the percentage of patients with atrial fibrillation prescribed oral anticoagulants or aspirin.