Background: We described the QTc interval prolongation and related adverse cardiac events during the administration of hydroxychloroquine (HCQ) and its combinations for treatment of COVID-19. Methods: The hospitalized patients who were infected with SARS-CoV-2 and received HCQ with initial and follow up ECGs from March 10th to May 30th were included. The critical QTc prolongation was accepted as QTc >500 ms if QRS<120ms and >550 ms if QRS >120 ms or ∆QTc levels >60 ms when compared with the initial ECG. Primary outcomes were critical QTc prolongation, ventricular tachyarrhythmia, and sudden cardiac arrest. Results: Out of 336 hospitalized patients with suspected or confirmed COVID-19, 297 received HCQ, and 94 met the inclusion criteria, and 66 cases were included in final analysis. The mean baseline QTc was 444.5 (sd= 39.5) ms. In total, 63% of the patients’ QTc levels increased under HCQ treatment and critical QTc prolongation occurred in 8 cases (12%) all of whom were male. The male gender (p=0.033), DM (p=0.035) and oseltamivir use (p=0.047) were significantly associated with critical QTc prolongation. In multivariate analysis, DM (OR:5.8, %95 Cl:1.11-30.32, p:0.037), and concomitant use of oseltamivir (OR:5.3, %95 Cl:1,02-28, p:0.047) were found to be associated with critical QTc prolongation. Conclusion: Critical QTc prolongation was detected in 12% of the patients. The DM and concomitant oseltamivir use were associated with critical QTc prolongation. The use of concurrent drugs that have potential to enhance QTc interval should be kept in mind and special attention should be paid for ECG monitoring.