Functional and structural changes in atrial muscle constitute a substrate for atrial fibrillation. The pathological changes in the left atrium decrease conduction velocity and result in prolongation of the P wave duration. The aim of the study was to assess the duration of the P wave in patients with paroxysmal and persistent atrial fibrillation. The study group consisted of 119 patients diagnosed with atrial fibrillation, 57 women and 62 men, aged 65.3+/-9.4 years. There were 65 patients with paroxysmal AF and 54 with persistent AF. In this group the electrical cardioversion was performed. The P wave duration, was measured using electrophysiological system in all leads at paper speed of 200 mm/s. The studied patients did not differ in term of age, gender and comorbidities. The patients with persistent AF had longer P wave duration (159.9+/-22.3 vs 144.6+/-17.2 ms, p<0.001), higher glucose concentration (119.4+/-33.4 vs 108.0+/-24.6 mg/dL, p=0.015). Those results were not influenced by the anti-arrhythmic treatment. The persistent atrial fibrillation shows prolongation of the P wave duration over the paroxysmal form of the arrhythmia, independently to age, gender and anti-arrhythmic medication. The prolongation of the P wave related to persistent arrhythmia should force the physicians to earlier restoration of the sinus rhythm in order to its more successful long term maintenance. Key words: P wave duration, atrial fibrillation, diabetes mellitus, chronic kidney disease