OBJECTIVES: Substantial overuse of newer broad-spectrum antibiotics has been a grave public health concern as it results in drug resistance and substantial healthcare costs. Little is known about how health insurance status is related with the prescribing of antibiotics at the adult primary care. We aim to assess the relationship between broad-spectrum antibiotic prescribing and patients’ insurance status in treatment of acute respiratory tract infections (ARTI) in adult primary care using a nationally representative sample. METHODS: We analyzed 2006 National Ambulatory Medicare Care Survey, for adult aged 18 years or older cared for ARTI. Those patients with a concomitant diagnosis with other common outpatient infections that might be treated with antibiotics were excluded.  Analysis was restricted to those patient visitation cared by physician specialty with general practice, family practice, and internal medicine. Multivariate logit regression analysis was performed to assess the relationship between patient insurance status and the prescribing of broad-spectrum antibiotics controlling for age, gender, race and ethnicity, physician specialty, and comorbidities. RESULTS: Of 851 adults patients care for ARTI, 38% were prescribed one or more broad-spectrum antibiotics. In multivariate regression analysis, compared to those with private insurance, those with Medicaid, a public insurance program for low-income Americans, was associated with lower likelihood of prescribing of broad-spectrum antibiotics (adjusted odds ratio (OR) 0.496, p=0.003), so were those without health insurance (adjusted OR 0.499, p=0.028), and those with Medicare, a public insurance program for the elderly or disabled adults (adjusted OR 0.666, p=0.091). CONCLUSIONS: In the case of ARTI, those with private insurance were substantially more likely to be prescribed with broad-spectrum antibiotics, where the society may be better off if such overuse of antibiotics could be reduced.