Background P-wave duration (P-DURATION) and P-wave area (P-AREA) have been linked to risk of atrial fibrillation (AF), but they do not improve the efficacy of Framingham AF risk score. We suggest the incorporation of both variables in one index, the P-wave area/P-wave duration (P-AREA(/)DURATION) index, which may be considered an expression of the average amplitude of the P wave that reflects aspects of P-wave morphology. Objective To assess the prognostic value of P-wave area/P-wave duration index (P(AREA/DURATION)index) in lead II together with other P-wave indices (PWIs) in incidence of AF in the Copenhagen Holter Study. Methods The study included 632 men and women, between 55 and 75 years with no apparent heart disease or AF. Baseline standard 12-lead Electrocardiography (ECGs) were analyzed manually. Results The median follow-up time was 14.7 (14.5;14.9) years. A total of 68 cases of AF and 233 cases of death were recorded. The restricted cubic spline method showed a U-shaped association between P(AREA/DURATION)and rate of AF. The lowest quintile of P(AREA/DURATION)index in lead II was associated with increased rate of AF, HR 2.80 (1.64-4.79). The addition of the new index to the Framingham model for AF improved the model in this population. The P(AREA)in lead II in its lowest quintile was also associated with increased rate of AF, HR 2.16 (1.25-3.75), but did not improve the Framingham model. P(DURATION)and P-wave terminal force (PTF) were not significantly associated with AF. Conclusion A flat P wave as expressed by a small P(AREA/DURATION)index in lead II is associated with increased rate of incident AF beyond known AF risk factors.