Atrial fibrillation (AF) is the most prevalent arrhythmia in the elderly, associated with high mortality and morbidity rates. Changes in electrocardiographic P waves were associated with AF in patient series, but the long-term prognostic value of P-wave characteristics in the development of AF was not shown. The aim was to evaluate P-wave duration and morphologic characteristics as potential independent risk markers for the long-term development of AF in the general population. A nested case-control design was adopted in subjects aged 55 to 74 years and apparently healthy at baseline. Baseline P-wave items of 40 patients who developed AF within the 10-year period were compared retrospectively with those of 120 matched controls. Broad maximum P waves (>/=120 ms) at baseline were observed in 70% of patients with AF and 41% of controls (p = 0.002). Maximum P-wave duration proved to be a significant risk marker independent of blood pressure, body mass index, and other electrocardiographic findings. However, this association seems overruled by the predictive value of morphologic changes defined as notched or deflected P waves (p = 0.0002). The joint occurrence of longer duration in combination with morphologic changes in the P wave proved a very important risk indicator for the development of AF over 10 years, with an adjusted odds ratio of 13.4 (95% confidence interval 3.3 to 46.6). In conclusion, enhanced clinical appreciation of P-wave items on the standard electrocardiogram at rest, preferably evaluated in all 12 leads, may help identify a group at high risk of the development of AF at an early stage.