Objective To evaluate the effects of "Prescription Automatic Screening System (PASS)" on adverse drug events (ADEs) and improvement of clinical safety in elderly patients.Methods Elderly in-patients (>65 years old) from Gerontology and Gastroenterology Departments,Dongfang Hospital Affiliated to Tongji Medical Universtiy from July 2007 to July 2009 were selected randomly.At phase Ⅰ,PASS was not used.At phase Ⅱ,PASS was used for patients from Gerontology Department,but not for patients from Gastroenterology Department.According to Naranjo score sheet,ADEs were determined.The incidence,distribution and severity of ADEs,and hospitalization duration and cost of preventable ADEs were compared between phase Ⅰ and phase Ⅱin the two departments.Results (1) The incidence of preventable ADEs and severe preventable ADEs decreased significanfly after PASS intervention (P<0.01) in Gerontology Department,but there was no significant difference in Gastroenterology Department (P>0.05).(2) The severe events accounted for a maximum of ADEs,which decreased most significantly after PASS intervention.(3) Hospitalization duration and hospitalization costs for preventable ADEs decreased significantly (P<0.01) in Gerontology Department after PASS intervention,but there was no significant difference (P>0.05) in Gastroenterology Department.Conclusion ADEs are common in elderly in-patients.The more severe the ADEs is,the more possible is it be prevented.For preventable ADEs,PASS intervention can achieve more clinical benefit,including significantly shortening the hospitalization duration,lowering hospital costs,reducing medical risks,reducing morbidity,and improving clinical safety.However,there are still some shortcomings.