Dependence of the corrected QT interval (QTc) duration on clinical signs of atrial fibrillation (AF) was studied on the episodes of sinus rhythm (SR) and AF paroxysms in 101 patients aged (62±10) years with persistent and paroxysmal AF (duration (7±6) years). While heart rate (HR) increased, QTc duration decreased during both, SR and AF episodes. While NYHA functional class (FC) of heart failure (HF) increased, QTc duration increased on the SR episodes (NYHA FC 0 – (409±15) ms; NYHA FC I – (421±33) ms; NYHA FC II – (420±26) ms; NYHA FC III – (424±33) ms; Р>0,05) and on the AF episodes in subgroup with intraventricular blockades (NYHA FC I – (400±23) ms; NYHA FC II – (421±30) ms; NYHA FC III – (422±39) ms). There was no dependence of QTc duration on arterial hypertension (AH) degree during SR and AF episodes. During SR episodes QTc duration was higher in patients with left ventricle (LV) ejection fraction (EF) < 55 %, than in those with LV EF ≥ 55 % ((437±28) and (415±24) ms, respectively; Р>0,05). During AF episodes dependence was vice versa – (417±35) and (419±24) ms, respectively (Р>0,05). QTc duration differences on the episodes of SR and AF in patients with paroxysmal and persistent AF were not statistically significant and were defined by clinical signs and ventricular blockades presence.