Objective - To determine the prevalence of anticoagulant (AC) treatment of patients with atrial fibrillation in primary health care. To identify complications in the same patients during 1 year. Design - Cross-sectional study and 1-year follow-up. Setting - Seven health centres with a total population of 164 093. Subjects - Five hundred and twenty-two anticoagulated patients with atrial fibrillation. Results - The age-adjusted prevalence of AC treated patients with atrial fibrillation was 0.30%. Of the 522 patients, 240 were men, mean age 69.6 years; and 282 women, mean age 75.1 years. At the beginning of the study 85% and after 1 year 81% of the latest prothrombin time values were within recommended range. After 1 year 414 out of the 522 patients continued AC treatment. During the 1-year follow-up 62 patients had minor or major complications. Eleven patients (2.1%) had to discontinue AC treatment because of complications. Prothrombin tests were mainly taken at 3-4 week intervals. Conclusion - High quality AC treatment is possible in the hands of general practitioners.