We used a high-resolution mid-IR tunable-laser absorption spectroscopy ͑TLAS͒ system with a single IV-VI laser operating near 5.2 m to measure the level of exhaled nitric oxide ͑eNO͒ in human breath. A method of internal calibration using simultaneous eNO and exhaled CO 2 measurements eliminated the need for system calibration with gas standards. The results observed from internally calibrating the instrument for eNO measurements were compared with measurements of eNO calibrated to gas

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... and were found to be similar. Various parameters of the TLAS system for eNO breath testing were examined and include gas cell pressure, exhalation time, and ambient NO concentrations. A reduction in eNO from elevated concentrations ͑ϳ44 parts in 10 9 ͒ to near-normal levels ͑Ͻ20 parts in 10 9 ͒ from an asthmatic patient was observed after the patient had received treatment with an inhaled glucocorticoid anti-inflammatory medication. Such measurements can help in evaluating airway inflammation and in monitoring the effectiveness of anti-inflammatory therapies.