Background: End Stage Renal Disease (ESRD) patients have an extremely high mortality and the leading cause of death is cardiovascular disease which accounts for 50% of all deaths. It is estimated that about one third is due to arrhythmias. Previous studies reveal a higher risk of various arrhythmias in dialysis patients but the prevalence is uncertain. Atrial fibrillation is the most common arrhythmia among patients with ESRD. The arrhythmia is often asymptomatic, but the risk of stroke increases dramatically and the annual mortality doubles. Autonomic cardiac dysfunction is often seen in patients with ESRD, and this is expressed by attenuated Heart Rate Variability (HRV) which is a measure of the variation in the time interval between heart beats. Attenuated 24 hours HRV is associated with an increased risk of sudden cardiac death in the general population and among patients with ESRD. N-3 polyunsaturated fatty acids (PUFAs) in fish or fish oil supplements have been shown to increase HRV and reduce the risk of various ventricular and supraventricular arrhythmias in some but not all studies, but this effect has only been sparsely investigated in the high risk patients with ESRD, who has a very low intake of n-3 PUFAs.