Purpose: Many medications induce QT prolongation and increase the risk of torsade de pointes. Concurrent use of two or more of these medications can further increase the risk. We investigated the frequency with which these medications are prescribed alone or in combination in a cohort of the patients of Kyungpook National University Hospital (KNUH) for 1 year. Methods: Using the patient database system of KNUH, we investigated retrospectively 193,266 subjects aged 15 years or older. After identifying 23 medications with QT-prolonging potential, we evaluated the frequency of prescription of these medications and overlapping prescriptions for two or more of these medications in 2008. Also we investigated the prevalence of obtainment a pretreatment electrocardiography (ECG) before prescribing the medications. Results: Nearly 11,286 subjects (5.04%) got 223,678 prescriptions for QT-prolonging medications. Of these, 805 subjects (7.13%) got overlapping prescriptions for two or more of the medications; 59 subjects (0.52%) got overlapping prescriptions for three or more of these medications. Seventeen percent of subjects who got overlapping prescriptions were aged 65 or older; 45.0% were women. Antidepressants were involved in nearly 44.5% of the cases. In 274 cases (3%), a pretreatment ECG was investigated. Conclusion: Overlapping prescription of QT-prolonging medications is common, and antidepressants are involved in 44.5% of these cases. Analysis of the prescription databases is useful for detecting potentially risky prescribing behaviors. An investigation of a pretreatment ECG and a follow-up ECG may help preventing overlapping prescriptions.