The relationship between the most common disease-causing mutations, the clinical manifestation, and lung function was prospectively assessed in 60 infants (33 females, 27 males) with cystic fibrosis at time of diagnosis (age: 7.2 months; range: 0.8-23.8 months). Lung function was assessed by infants whole-body plethysmography. Age at time of diagnosis was independent from the genotype. Weight gain from birth until the time of diagnosis expressed in percent predicted of a normal population was lower in the 3905insT group (57.9 +/- 19.0%) compared with deltaF508 homozygotes (62.5 +/- 20.6%; n.s.) and the R553X group (85.9 +/- 10.9%; p < 0.005). Differences regarding lung function within the genetic groups are mainly related to pulmonary hyperinflation, measured by thoracic gas volume (TGV), present in 8 of 9 infants with 3905insT, differentiating this frameshift mutation (TGV of 7.0 +/- 3.6 SD-S) from the R553X mutation (TGV 2.1 +/- 4.6 SD-S; p < 0.02). It is concluded that the variable disease findings in infants with cystic fibrosis is clinically and functionally reflected by features already present at time of diagnosis. The degree of pulmonary hyperinflation is, at least partly, influenced by the genotype.