OBJECTIVES To investigate trends in the prevalence of obesity and annual medical costs in normal and obese patients with asthma from 2001-2006. METHODS This cross-sectional analysis utilized data from the 2001-2006 Medical Expenditure Panel Surveys. Self report of a diagnosis of asthma or ICD-9-CM code: 493, with exclusion of patients with pregnancy, malignancy, kidney dialysis, immunodeficiency, age <18 or ≥75 years old, or body-mass-index(BMI) of <18.5, identified 10,402 asthma patients. Patients with BMI's of ≥30 and 18.5 to <25 were classified as obese and normal, respectively. Medical costs included all treatment costs, except dental or injury costs, with costs related to the respiratory system being those associated with ICD-9 codes 460–516. Bootstrap method was used to calculate the standard error (SE) of medical costs, while t-test was used to compare medical costs. All costs were converted to 2006 U.S. dollars. Data were analyzed using SAS and STATA. RESULTS Age adjusted prevalence of obesity was 25.9% and 26.4% among the total population, but 35.1% and 35.6% among asthma patients in 2001 and 2006, respectively. While average medical costs in asthma patients with normal BMI maintained a steady level from $2911 (SE:$492) in 2001 to $2630 (SE:$286) in 2006, those costs increased by 30% from $4442 (SE:$371) in 2001 to $ 5776 (SE:$401) in 2006 (P=0.0101) in obese asthma patients. Costs related to the respiratory system in obese asthma patients did not change over time, with increases in medication costs but decreases in inpatient costs. CONCLUSIONS Even though the prevalence of obesity remained consistent from 2001 to 2006, the average medical cost in asthma patients with obesity increased significantly, suggesting an upward trend in the economic burden of obesity. Effective strategies to prevent or reduce obesity should be introduced as a cost-effective intervention in asthma patients.