The optimal strategy for monitoring cystic fibrosis lung disease in infancy remains unclear. Our objective was to describe longitudinal associations between infant pulmonary function tests, chest radiograph scores and other characteristics. Cystic fibrosis patients aged ≤24 months were enrolled in a 10-centre study evaluating infant pulmonary function tests four times over a year. Chest radiographs ∼1 year apart were scored using the Wisconsin and Brasfield systems. Associations of infant pulmonary function tests with clinical characteristics were evaluated with mixed effects models. The 100 participants contributed 246 acceptable flow/volume (forced expiratory volume in 0.5 s (FEV0.5) and forced expiratory flow at 75% of the forced vital capacity (FEF75%)), 303 functional residual capacity measurements and 171 chest radiographs. Both Brasfield and Wisconsin chest radiograph scores worsened significantly over the 1-year interval. Worse Wisconsin chest radiograph scores and Staphylococcus aureus were both associated with hyperinflation (significantly increased functional residual capacity), but not with diminished FEV0.5 or FEF75%. Parent-reported cough was associated with significantly diminished forced expiratory flow at 75% but not with hyperinflation. In this infant cohort in whom we previously reported worsening in average lung function, chest radiograph scores also worsened over a year. The significant associations detected between both Wisconsin chest radiograph score and S. aureus and hyperinflation, as well as between cough and diminished flows, reinforce the ability of infant pulmonary function tests and chest radiographs to detect early cystic fibrosis lung disease.