Nitric oxide (NO) can be detected in exhaled gas in human subjects. It is produced by nitric oxide synthase (NOS) and is rapidly metabolized to nitrite and nitrate (NO₂/NO₃). Exhaled NO is reported to be elevated in patients with asthma, bronchiectasis, or upper respiratory tract infection. Recent reports have shown no increase of exhaled NO in stable cystic fibrosis (CF). We hypothesized that NOS activity is increased in patients with acute pulmonary exacerbation of CF. We therefore measured exhaled NO and sputum NO₂/NO₃ in three subject categories: patients with acute pulmonary exacerbation of CF, patients with stable CF, and healthy control subjects. Mean ± SD exhaled NO was significantly higher in control subjects (8.8 ± 4.9 ppb) than in both acute (3.8 ± 3.9 ppb) and stable (5.0 ± 2.5 ppb) patients. Sputum NO₂/NO₃ was significantly higher in acute patients (774 ± 307 μ mol/L) when compared with both stable patients (387 ± 203 μ mol/L) and control (421 ± 261 μ mol/L) subjects. Sputum NO₂/NO₃ did not return to normal in a subgroup of patients assessed after 2 wk of intensive antibiotic and glucocorticoid treatment. These results confirm that exhaled NO is not a useful measure of airway inflammation in CF. Elevated levels of sputum NO₂/NO₃ suggest that NOS is activated during acute pulmonary exacerbations of CF.