Purpose: To determine the extent and rate of prescription drug therapy, especially polypharmacy and the prevalence of potentially inappropriate medication use in Korean elderly ambulatory patients based on an explicit criterion. Methods: Performed a retrospective study of 65 years or older ambulatory patients visiting a university hospital based clinic from January 2002 to April 2004. Study determined the patterns of drug prescription per Anatomical Therapeutic Chemical Classification and the potentially inappropriate medication usage based on explicit Beers criteria. Results: Of the 4,042 elderly patients the mean number of prescription was $2.2{pm}2.0$, which was similar between genders and all age groups within the elderly. 10.7% of patients were prescribed with more than 5 medications concurrently. The most frequently prescribed medication was the drugs used for treating nervous system diseases (44.3%), followed by alimentary tract/metabolism disorders (27.6%), cardiovascular disease (10.7%), blood/blood forming disorders (4.3%), respiratory disorders (6.5%), and musculoskeletal diseases (3.2%). A total of 511 elderly (13%) was prescribed with medication that met the criteria for=1 potentially inappropriate drugs for the elderly. This proportion was similar between genders and all age groups within the elderly. Among these 511 elderly patients the mean number of potentially inappropriate drugs prescribed was $5.1{pm}3.3$ drugs. Potentially inappropriately prescribed drugs included amitriptyline (76 cases), diazepam (69 cases), ketorolac (57 cases), short acting nifedipine (44 cases), triazolam (38 cases), and hydroxyzine (38 cases). Conclusion: Potentially inappropriate drug prescribing in Korean ambulatory elderly patients are common. Education programs and interventions aimed at optimizing the prescribing and dispensing of the most appropriate drugs are needed.