Adverse drug reactions (ADR) may result in increased hospital admissions, morbidity and mortality, adding extra cost to healthcare expenditures. Thus, it is critical to activate ADR monitoring and reporting program in tertiary hospitals in developing countries such as Korea. This study was performed to identify the types of ADR being reported in a tertiary hospital, Samsung Medical Center, and to find out the ways to improve current ADR monitoring system. Of 464 ADR reports submitted to the pharmacy department during the 6-month survey period, $97.8\%$ of the reports were from out patient and $48.5\%$ were from patients aged between 50 and 60. The medical department with the highest frequency in ADR reporting was Internal Medicines $(35.6\%)$. The most common ADR manifestations were gastrointestinal complaints $(43.4\%);and;75\%$ of the reported cases were mild in their severity. The most common drugs suspected of causing ADR were CNS drugs which accounted for $32.8\%$. In terms of causality assessment, $85.1\%$ of the reports were probable cases by WHO causality assessment criteria. In regards to sources of report, $75.6\%$ of ADR were reported by physicians and $24.4\%$ by nurses. There were no ADR reported by pharmacists. In conclusion, there is an urgent need to improve ADR monitoring system for inpatient and to motivate pharmacist involvement in ADR monitoring and reporting in Korea.