Patients treated with haemodialysis are at high risk of sudden cardiac death (SCD) often caused by arrhythmias. Atrial fibrillation (AF) is frequent among haemodialysis patients and is associated with increased mortality. Prolonged QTc is a risk marker of ventricular arrhythmia and is thereby associated with SCD. Studies have suggested that n-3 PUFA may have an antiarrhythmic effect, but the exact mechanism is not clear. The aim of this study was to examine whether AF was associated with n-3 PUFA in plasma phospholipids and whether supplementation with n-3 PUFA would shorten the QTc interval in haemodialysis patients compared to placebo. In a double-blinded randomised, placebo-controlled intervention trial 206 haemodialysis patients with CVD were treated with 1·7 g n-3 PUFA or placebo (olive oil) daily for 3 months. Blood samples and electrocardiogram evaluations were carried out at baseline and after 3 months. The QT interval, PQ interval and heart rate were measured in all patients with sinus rhythm (SR). At baseline 13 % of patients had AF. The content of the n-3 PUFA, DHA, was significantly lower (P < 0·05) in serum of patients with AF compared with patients with SR. Thus, the DHA content was independently negatively associated with AF. Supplementation with n-3 PUFA did not shorten the QT interval significantly compared to the placebo group (P = 0·42), although subgroup analysis within the n-3 PUFA group revealed a shortening effects on QTc (P = 0·01). In conclusion, an inverse association was found between the presence of AF and the plasma DHA in haemodialysis patients. Intervention with n-3 PUFA did not shorten the QTc interval compared to placebo.