Objective: The estimation of budget impact is important in listing a new drug, but there are a lot of uncertainties. We analyzed usage of new drug after reimbursement and investigated various factors influencing budget impacts of the new drug to get a guidance for public insurance BIA in Korea. Methods: We used 3 year claims data of 23 new drugs listed in 2004 to analyze usage pattern and market share. We evaluated influencing factors that clinical improvements, treatment cost, disease burden, patient number, market competition, type of company, etc. and conducted multiple regression analysis using these factors. Results: The indications of the 23 listing drugs were for cancer, hypercholesterolemia, diabetes, schizophrenia, pneumonia, peptic ulcer, rheumatoid arthritis, hepatitis B, HIV treatment, etc. At third year after new drug listing, average market share incrementally rose to 20%(0.06-78%, range) both in patient number and volume of use. In case of the new drug with clinical improve and higher cost, the average market share amounted to 33%(n=4). The market share of drugs with no improve and lower costs amounted to 26%(n=8). When total patient number of new and pre-listed drugs were under 50,000, market share of new drugs amounted to 25%, 35% of total volume and patient number, respectively. But in case of over 250,000 of patients, market share of new drugs were less than 10% in both. New drugs commanded 27%, 4%, 0.08% of market when number of pre-listed competing drugs were <25, 5-25, 25Conclusion: Clinical improvements, disease burden, number of patients, number of pre-listed drug, and company type may affect to market diffusion of new drugs. So we suggest these results be considered in forecasting future usage of new drug and conducting budget impact analysis.