Abstract: AIM:To evaluate whether early prophylaxis with calcium channel blocker reduces the incidence and morbidity associated with atrial arrhythmias after major thoracic surgery. METHODS: In this randomized, double-blinded and placebo-controlled study, 341 patients were given diltiazem or placebo orally before and immediately after lobectomy (169/172) and thereafter for 14 days. The primary end point with respect to efficacy was a sustained ≥15 min or clinically significant atrial arrhythmia during treatment. RESULTS: Postoperative atrial arrhythmias incidence in the diltiazem group was significantly lower compared to the placebo group, with nearly a 50% reduction of the incidence of clinically significant arrhythmias. The maximal ventricular rate during AF in the diltiazem group was significantly lower than in the placebo group. No significant difference was observed in the incidence of postoperative complications and the length of hospital stay between groups. There were no serious adverse effects caused by diltiazem. CONCLUSION: Prophylactic diltiazem reduces the incidence of clinically significant atrial arrythmias in patients at high risk for this complication after major thoracic operations.