Objective: Some precursor P wave changes on electrocardiogram (ECG) before the atrial fibrillation (AF) episodes occur in the hyperthyroidism. Our aim was to compare the effect of two antithyroid drugs (ATD) on P wave duration and dispersion (PWD) in patients with hyperthyroidism. Methods: Fifty patients (13 men, 37 women; mean age 39.2±13.2 years) with newly diagnosed overt hyperthyroid patients with Graves’ disease (GD) were enrolled in the prospective, randomized study. The maximum P wave duration (Pmax) and the minimum P wave duration (Pmin) were measured in all 12-lead surface ECGs. The patients were consecutively randomized to propylthiouracil (PTU) (n=24) and methimazole (MMZ) (n=26) groups. Electrocardiogram was repeated within euthyroid state after the 18-month ATD treatment. Student t-test, Mann-Whitney U and Pearson Chi-square tests were used for comparisons of the data between groups. The differences between pre- and post-treatment measurements within groups were evaluated by Wilcoxon Sign Rank test. The correlation of data was tested by using Spearman correlation analysis. Results: The maximum P wave duration (Pmax) was 90 (80-110) and 90 (90-110) msec, (p=0.586), and PWD was 35 (22.5-48.7) and 40 (30-40) msec, respectively (p=0.952) in PTU and MMZ groups. After euthyroidism was achieved, Pmax was 80 (80-90) and 87.5 (80-90) msec (p=0.676), and PWD was 27.5 (20-35) and 27.5 (20-30) msec in PTU and MMZ groups, respectively (p=0.540). After ATD treatment PWD decreased (p=0.009 and p<0.001, respectively) in both of PTU and MMZ groups. However effects of ATD on PWD change were similar (p=0.486). Conclusion: P wave duration and PWD are found to be prolonged in hyperthyroid patients with GD. Both propylthiouracil and methimazole reduce the P wave duration and dispersion. Thus, we can conclude that improvements in atrial conduction properties are not associated with the type of ATD but with only achievement of euthyroidism.