[영문]Objective: This study evaluated the effect of a Computerized Physician Order Entry system with basic Clinical Decision Support (CPOE/CDSS) on the incidence of medication errors (MEs) and preventable adverse drug events (pADEs). Design: Interrupted time-series design. Measurements: The primary outcome measurements comprised the percentage of medication orders with one or more MEs and the percentage of patients with one or more pADEs. Results: Pre-implementation, the mean percentage of medication orders containing at least one ME was 55%, whereas this became 17% post-implementation. The introduction of CPOE/CDSS has led to a significant immediate absolute reduction of 40.3% (95% CI: ?45.13%; ?35.48%) in medication orders with one or more errors. Pre-implementation, the mean percentage of admitted patients experiencing at least one pADE was 15.5%, as opposed to 7.3% post-implementation. However, this decrease could not be attributed to the introduction of CPOE/CDSS: taking into consideration the interrupted time-series design, the immediate change was not significant (?0.42%, 95% CI: ?15.52%; 14.68%) because of the observed underlying negative trend during the pre-CPOE period of ?4.04% [95% CI: ?7.70%; ?0.38%] per month. Conclusions: This study has shown that CPOE/CDSS reduces the incidence of medication errors. However, a direct effect on actual patient harm (pADEs) was not demonstrated.