A reliable risk factor for sudden cardiac death (SCD) for the general population remains to be defined. We propose the omega-3 index, defined as the combined percentage of eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) in red blood cell membranes. It reflects the EPA + DHA status of a given individual. It can be determined by a standardised and reproducible laboratory procedure. Several lines of evidence support the omega-3 index as a risk factor for SCD: in epidemiological studies, a steep dependence of risk for SCD and the omega-3 index has been observed between 6.5% (risk 0.1) and 3.3% (risk 1.0). EPA + DHA are antiarrhythmic on the supraventricular and ventricular levels. Dietary EPA + DHA reduce the incidence of SCD. Cardiac societies recommend EPA + DHA for prevention of SCD. The omega-3 index can assess risk for SCD and monitor therapy with EPA + DHA. Moreover, it compares very favourably with other risk factors for SCD.