Plasma neutrophil elastase-alpha 1 antiproteinase complex, lactoferrin and C-reactive protein (CRP) were determined over a 15-month period in 26 patients with cystic fibrosis, of whom 21 were chronically infected with Pseudomonas aeruginosa. Median concentrations of both neutrophil products and CRP were greater in patients who were clinically stable than in healthy subjects without cystic fibrosis. CRP concentrations increased further at the onset of symptomatic exacerbations. Thirty-five courses of intravenous antibiotics and 22 courses of oral ciprofloxacin were reviewed and revealed similar improvements in clinical scores and lung function tests for both forms of treatment. Intravenous antibiotics reduced the plasma concentrations of both neutrophil products and CRP, while oral ciprofloxacin only significantly reduced the concentration of neutrophil elastase-alpha 1 antiproteinase complex. Plasma concentrations of inflammatory markers were significantly greater in exacerbations associated with fever and leukocytosis. Statistical modelling demonstrated negative within-patient relationships between lung function and both CRP and lactoferrin, and positive relationships between the three inflammatory markers. Neutrophil granule products and CRP reflect the pulmonary inflammatory state in cystic fibrosis and may be of value in monitoring treatment.