Proinflammatory cytokines in sputum are useful markers of the activity of lung disease in cystic fibrosis (CF). Tumour necrosis factor alpha (TNF-alpha) and interleukin-8 (IL-8) concentrations in sputum of 10 CF patients were determined during exacerbation and IL-8 in sputum of 48 patients at a yearly follow-up when patients were in optimal clinical condition. In 9 patients of the former group, TNF-alpha levels were increased during exacerbation. In 4 patients, the peak occurred within 2 d (median value > 1500 ng/l), whereas the remaining 5 had peak values on days 3-6 (median value 720 ng/l). IL-8 levels were > 800 microg/l in all 10 patients, and in 9 cases there was a positive correlation between IL-8 and TNF-alpha. Baseline IL-8 levels of 48 patients showed considerable variation (median 207 microg/l, range 1.5-392). There was a significant correlation between IL-8 concentrations and current colonization with either Pseudomonas aeruginosa or Staphylococcus aureus in the lower airways (p = 0.002), immunoglobulin G levels (p = 0.02) and the severity of the pathological findings shown by chest X-ray (p = 0.008). High IL-8 and TNF-alpha values correlated with symptoms of deterioration. IL-8 levels seemed to be markers of both current bacterial colonization and the degree of lung damage.