Augmented sympathoadrenal activity during exercise may contribute to occurrence of various arrhythmias including atrial fibrillation (AF). The prolongation of intraatrial and interatrial conduction times and inhomogeneous propagation of sinus impulses are well-known characteristics of the atrium prone to fibrillate and are evaluated by maximum P-wave duration (P max), P-wave dispersion (PWD). To show the increased P max and PWD values in patients experiencing AF during exercise testing and the role of beta blockade on treatment of exercise-induced AF, 22 of these patients were compared with a control group consisting of 41 patients without AF attacks. P max (p=0.001) and PWD (p=0.001) values were significantly higher in patients with AF compared to those without AF. The development of AF during exercise testing was found to be positively correlated with P max (r =0.87, p<0.001), PWD (r =0.83, p=0.001), and work load (r =0.34, p=0.002) and negatively correlated with ejection fraction (r =-0.26, p=0.02). After the treatment with beta-blocking agents for 2 weeks, the decrease in P max and PWD values was accompanied by a much lower prevalence of exercise-induced AF. Consequently, the patients with AF had greater P max and PWD values compared to control subjects, and these simple parameters were well correlated with the occurrence of AF during exercise testing. Furthermore, treatment of these patients with beta blockers would appear to decrease the recurrence of exercise-induced AF and to be associated with a decrease in P-wave durations. ©2007 Sage Publications.