The patients. Randomisation. Patients with persistent AF will be recruited from clinic attendees and in-patient hospital patients at Ninewells Hospital and Medical School. A total of 150 patients with AF of more than 7 days duration and scheduled for elective direct current cardioversion will be recruited. The patients will be randomised to receive fish oil supplements (3g/day) or placebo on recruitment for a period of 4 weeks prior to cardioversion and continued after cardioversion until recurrence of AF or until the end of 6 months. All patients will be anticoagulated routinely. Patients on anti-arrhythmic drugs, left atrial size >6 cm, significant mitral valve disease, myocardial infarction in the last 3 months, unstable angina, NYHA IV heart failure, cardiac surgery in the previous 3 months, acute reversible conditions, significant thyroid, hepatic, pulmonary disease, pregnancy or child bearing potential will be excluded from the study.