Disorders of the respiratory system, such as cystic fibrosis (CF), involve the infiltration and activation of airway inflammatory cells, including neutrophils. This leads to the secretion of peroxidases, which react further with substrates in solution to produce oxidative metabolites, such as 3-chlorotyrosine. Elevated levels of modified tyrosine residues in the airways of patients with CF may be detectable by nuclear magnetic resonance (NMR) in correlation with inflammatory cell influx. In this study, high-resolution (500 MHz) 1H NMR was used to analyze the production of modified tyrosine residues resulting from in vitro stimulation of peripheral blood eosinophils and neutrophils, as well as in sputum samples from control subjects and patients with CF. Following in vitro stimulation, purified peripheral blood neutrophils generated 3-chlorotyrosine, while eosinophils produced predominantly 3-bromotyrosine and 3,5-dibromotyrosine. Chlorinated and brominated tyrosine residues were detected in sputum samples from patients with CF (N=7), but were not detected in the control group (N=9). Neutrophil counts in CF sputum correlated strongly with the presence of 3-chlorotyrosine (r2=0.869). Our findings indicate that neutrophil and eosinophil activation in CF is detectable by NMR. NMR may be a useful tool for the detection of biological markers of inflammatory processes in patient airways.