Nitric oxide (NO) is known to be present in measurable quantities in the exhaled air of normal subjects and at higher concentrations in asthmatic subjects not treated with glucocorticoids. We confirmed these findings by analyzing the mean mixed expired NO concentrations of 43 stable asthmatics and 90 normal subjects; NO levels were higher in the asthmatic population (13.9 parts per billion [ppb] versus 6.2 ppb, p < 0.001). Although the effects of glucocorticoids on the NO content of mixed expired air are known, it is not known if beginning systemic glucocorticoid therapy reduces exhaled NO levels in a given individual. To examine this question, seven patients needing emergency therapy for asthma underwent repeated measurements of mixed expired NO levels during their course of treatment with glucocorticoids. All patients had a reduction in mixed expired NO concentration (p = 0.002) and an accompanying improvement in airway obstruction. The decrease in exhaled NO was evident as early as 48 h after the initiation of therapy (p = 0.05). These data suggest mixed expired NO concentrations may prove useful as an index of asthma severity and treatment efficacy for an individual patient.