This study measures seriousness of adverse drug reactions (ADRs) among Korean physicians and pharmacists using two times surveys based on Delphi technique. Each participant scored 20 ADR terms on a scale of one to ten (10 being the most serious). We repeated the exercise for the 49 first survey respondents and 32 re-evaluated score. We compared the results of our survey with those of WHO CIOMS (Council for International Organization of Medical Sciences) working groups members conducted in 1995. The overall mean ADR seriousness score was 6.49 for Koreans and 5.12 for WHO CIOMS members, presenting Korean experts perceived more seriously for each ADR. Mean score changes for the same respondents showed similar trends regardless of access to the first survey results. There were no statistically significant score differences between the physicians and the pharmacists. The high consensus of seriousness for each ADR between the Korean experts and the WHO CIOMS members implies that the similar results are reproducible, suggesting the possibility of developing standardized tools for measuring the seriousness of individual ADRs in the future.