Observational study in 6 psychiatric hospitals in Flanders. Patients are included when a QT-prolonging drug is added to a medication profile that already contains a potential QT-prolonging drug. An ECG is taken before the administration of the new drug and a week after starting the new drug to investigate the change in duration of the QTc-interval. Risk factors for developing QT-prolongation and blood concentrations of potassium and creatinine are documented.