C-reactive protein (CRP) concentrations were evaluated in 9 cystic fibrosis (CF) patients with acute pulmonary exacerbations and 14 patients with acute exacerbations of asthma without any symptoms of an acute infection. CRP concentrations were serially evaluated over the course of therapy in CF patients and compared with pulmonary function tests (PFTs) and clinical scores. CF patients were treated with aerosolized bronchodilators, intravenous fluids, and chest physiotherapy for 48 hours. Intravenous antibiotic therapy was added after 48 hours. Initial CRP concentrations differed significantly between patients with CF and those with asthma. CRP concentrations were elevated in 7 of 9 CF patients versus 3 of 14 asthma patients (P < 0.02). In CF patients, CRP concentrations did not correlate with PFTs (except on day 0) or clinical scores. Frequently PFTs and clinical scores continued to improve after CRP levels had reached their lowest concentrations. CRP concentrations decreased only after the addition of antibiotic therapy.