OBJECTIVES: To assess whether screening improves the detection of atrial fibrillation (cluster randomisation) and to compare systematic and opportunistic screening. DESIGN: Multicentred cluster randomised controlled trial, with subsidiary trial embedded within the intervention arm. SETTING: 50 primary care centres in England, with further individual randomisation of patients in the intervention practices. PARTICIPANTS: 14,802 patients aged 65 or over in 25 intervention and 25 control practices. INTERVENTIONS: Patients in intervention practices were randomly allocated to systematic screening (invitation for electrocardiography) or opportunistic screening (pulse taking and invitation for electrocardiography if the pulse was irregular). Screening took place over 12 months in each practice from October 2001 to February 2003. No active screening took place in control practices. MAIN OUTCOME MEASURE: Newly identified atrial fibrillation. RESULTS: The detection rate of new cases of atrial fibrillation was 1.63% a year in the intervention practices and 1.04% in control practices (difference 0.59%, 95% confidence interval 0.20% to 0.98%). Systematic and opportunistic screening detected similar numbers of new cases (1.62% v 1.64%, difference 0.02%, -0.5% to 0.5%). CONCLUSION: Active screening for atrial fibrillation detects additional cases over current practice. The preferred method of screening in patients aged 65 or over in primary care is opportunistic pulse taking with follow-up electrocardiography. TRIAL REGISTRATION: Current Controlled Trials ISRCTN19633732 [controlled-trials.com].