Adverse drug events (ADEs) contribute significantly to patient morbidity and mortality as well as to cost for healthcare providers and society. Since only serious ADEs will result in hospitalisation, the evaluation of ADEs leading to hospitalisation reflects safety and appropriateness of ambulatory prescribing. ADEs occurring during hospitalisation may result from the special clinical situation of hospitalised patients; high degree of severity of diseases (e.g., renal failure, necessity of intensive care) and specific drugs administered only in hospitals. Moreover, the number of diagnostic and therapeutic procedures carried out daily puts the hospitalised patient at an extremely high risk for ADEs.