OBJECTIVES: To investigate the trend of treatment costs for asthmatic outpatients in Taiwan from 1998 to 2002. METHODS: Claimed database from Bureau of National Health Insurance of Taiwan's whole population were used to compute the total treatment costs and drug related costs on asthmatic outpatients during the year 1998 to 2002. Claims with ICD-9-CM code of asthma were obtained and then sorted per person to calculate the sum of treatment costs per person per year and the ratio of drug costs per total treatment costs. RESULTS: The following present the yearly data from 1998 to 2002: 1) mean drug costs per person per year: NT$.1286, 1486, 1873, 1940 and 2114; 2) mean of days per person per year: 26, 29, 36, 40 and 43 days; 3) total treatment costs of asthma : NT$. 4.3 billion, 2.9 billion, 2.4 billion, 2.5 billion and 2.7 billion; and 4) the ratio of drug costs/total costs: 54.92%, 56.48%, 58.15%, 58.52% and 58.84%. CONCLUSIONS: Drug therapy for patients with asthma play an important role in asthmatic care. In Taiwan, the study indicated that drug costs per person per year is growing every year. Future research will investigate the appropriateness of these drug therapy and posible drug savings in the future.