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Exhaled carbon monoxide is a useful marker of airway inflammation in untreated asthma. Whether exhaled CO is clinically useful in steroid treated patients in a hospital setting is uncertain. We therefore studied exhaled CO as a marker of asthma severity in clinical practice. Non-smoking “acute ” asthmatics (hospitalised; n=33), “stable ” asthmatics (n=35), and healthy controls (n=22) were recruited. Exhaled CO, peak expiratory flow (PEF) and FEV1 were measured daily (hospitalised cases) or once only (stable outpatients). Inpatients were managed without knowledge of the results. Exhaled CO levels in acute asthmatics (initial levels), stable asthmatics and controls were similar (median=2.0 ppm, h=5.05, p=0.08). In acute asthmatics, initial exhaled CO did not correlate with duration of hospitalisation, doses of intravenous corticosteroids, doses of nebulised salbutamol, PEF ( % predicted) or FEV1 ( % predicted). In stable asthmatics, exhaled CO did not correlate with corticosteroid dosage, PEF ( % predicted) or FEV1 ( % predicted). In the setting of acute hospitalised asthma patients, exhaled CO may not add any further to clinical management. This may in part be due to prior treatment with corticosteroids