In 15 patients with cystic fibrosis 18 blood samples were investigated for signs of infection including full white blood count, c-reactive protein (CRP) and tumour-necrosis-factor alpha (TNF). Ten patients were hospitalized for pulmonary exacerbation, one for orthostatic collapse and one for equivalent of meconium ileus. The latter two as well as three out-patients with cystic fibrosis on routine-visits served as controls. Blood was taken on admission and at the time of the visit in our out-patient department, respectively. In three cases, blood was taken repeatedly during their stays in hospital. While leucocytosis (17.700 +/- 3.500) and elevated CRP-levels (6.4 +/- 7.3 mg/dl) pointed to an infectious cause of deterioration in the exacerbation-group, TNF-levels without exception were undetectable (less than 15 pg/ml). In the control group, leucocyte counts (10.700 +/- 3.600) and CRP-levels (1.2 +/- 1.1 mg/dl) showed minor pathologic results. TNF-levels were undetectable, too. While elevated TNF-levels measured quantitatively in patients with invasive bacterial infections, e.g. septicaemia due to Neisseria meningitidis, correlate well with prognosis of disease, in patients with cystic fibrosis such a relationship can't be found.