The adverse drug events (ADE) is not only common but also expensive. Although it was expected that ADE could be prevented by using computer-based clinical decision support system (CDSS), it is not widely accepted in the clinical field. Therefore the purpose of this study was to verify whether CDSS can reduce ADE by meta-analysis. We searched literatures by Medline from 1975 to 2002 with key words of clinical decision support system, medication error, and adverse drug event. We also searched references of review articles as well as textbooks on medical informatics. The criteria for quality evaluation were as follows: 1) the objec t were physician, nurse, pharmacist, 2) case design for CDSS analysis was pe rformed random c linical te st of experimental-control group, 3) deal with a adverse drug event organization whether or not. Among 290 retrieved articles five studies were selected for quantitative meta-analysis. The overall effect size of the risk of adverse drug event due to CDSS was calculated by common odds ratio using MetaKorea (http://www.metakorea.or.kr). Before the integration of each effect sized into common eff ect sizes the homogene ity test were conducted. All studies were ca se control design and cases were ADEs. Homogenity of studies were conducted by Mantel-Haenszel method. The chi-square is 10.78 (p<0.05). For evaluation of odds ratio, random effec t model was used. The overall odds ratio of CDSS associated with ADE was 0.315201 (95% confidence interval = 0.191411-0.519049). Our result suggested a negative association between use of CDSS and the development of serious ADE. So we concluded that the development of serious ADE was reduced using CDSS.