Airway inflammation in children younger than 5 yr of age is difficult to assess, particularly in patients with cystic fibrosis (CF). Furthermore, determining responses to therapies is often subjective in infants, especially those with CF. To determine whether airway DNA levels could be used as an index of airway inflammation, we measured DNA levels in bronchoalveolar lavage fluid (BALF), using a Hoechst dye-binding assay. BALF DNA levels and neutrophils from 16 infants with CF were compared with levels obtained from seven older CF patients and nine control children who underwent bronchoalveolar lavage for evaluation of other pulmonary diseases. BALF DNA was increased in both infants (3.2 +/- 0.7 microg/ml) and older patients with CF (5.4 +/- 0.9 microg/ml) compared with the controls (0.7 +/- 0.2 microg/ml) (mean +/- SEM). BALF DNA levels were not significantly different between infants and older patients with CF. BALF neutrophil counts in CF patients were significantly higher than in controls. Furthermore, BALF DNA levels and total neutrophil counts in infants with CF correlated positively with one another. We conclude that: (1) DNA levels were easily quantifiable in BALF of young children; (2) DNA levels in BALF from CF patients were greater than in a group of children with other pulmonary diseases, and that in some infants with CF, BALF DNA levels were equivalent to those of much older patients with CF; (3) DNA levels in BALF correlate with BALF neutrophil number, an index of inflammation; and (4) some infants with CF have increased levels of DNA in BALF, which may justify a clinical trial of aerosolized rhDNase in this population.