Background: A pharmacist`s participation in medical rounds in intensive care unit (ICU) is becoming popular nowadays. In this study, we investigated the effect of pharmacologic intervention by a pharmacist`s participation in medical round in ICU on prevention of adverse drug events (ADEs). Methods: From March 2011 to July 2011, the intervention data were obtained by participating in medical round two or three times a week, and by reviewing electronic medical records of patients admitted to surgical ICU. The incidence, cause, and type of ADEs were noted, respectively. Expected cost avoidance was calculated from interventions, which were considered to be preventive of ADEs. The acceptance rate of pharmacologic interventions was noted. Results: Among 2781 patients, a total of 159 intervention data were collected in 90 patients. Recommendation for drug dosage adjustment or monitoring in patients with potential overdose and sub-therapeutic dose made up 82% of the total interventions. In 8% of interventions, initiation of drug therapy was recommended. 83% of the interventions were accepted and the acceptance rate of interventions within 24 hrs was 58%. The rate of the interventions, which were considered to be preventive of ADEs was 62%. Expected cost reduction obtained by preventing ADEs was 25,867,083￦ during a 5-month period. Conclusions: A pharmacist`s participation in physician rounds in ICU was associated with prevention of ADEs and subsequent reduction of the cost in drug therapy.