PURPOSE: To estimate the prevalence of co-prescribing contraindicated drugs for elderly patients in Busan. METHODS: We used the Health Insurance Review Agency (HIRA) claims database. Study population consisted of elderly patients who visited clinics or hospitals in Busan metropolitan city from January 1, 2000 to December 31, 2001. Contraindicated drugs were defined as 162 combinations of contraindicated drugs announced by the Korea Ministry of Health and Welfare in 2004. The co-prescription of contraindicated drugs was defined as prescribing two or more contraindicated drugs in combination in the same prescription. The prevalence of co-prescribing contraindicated drugswas estimated as proportion of co-prescribed patients out of the study patients. We estimated and age-adjusted prevalence and its 95% confidence interval of co-prescription of contraindicated drugs among the elderly patients in Korean population in 2001. RESULTS: The study elderly patients were 262,952 with 2,483,227 prescriptions. Among the study patients 1,208 (4.6%) were prescribed contraindicated drugs in combination. A total of 16,255 patients were estimated as the number of co-prescribed patients among the Korean elderly in 2001. Age-standardized prevalence of co-prescription to the Korean elderly was estimated to be 45 per 10,000 persons. The most frequently prescribed combinations were cisapride & amitriptyline, roxithromycin & ergoloid mesylate, and terfenadine & erythromycin, and the frequency were 325 (16.8%), 149 (7.7%), and 132 (6.8%),respectively. CONCLUSIONS: The contraindicated drugs were co-prescribed to the elderly patients in Korea. Many of these co-prescriptions should be avoided if unnecessary. The patients should be carefully monitored if they were inevitably prescribed the contraindicated drugs.