Introduction Respiratory infections in childhood have been related to reduced adult lung function, but few studies have examined the timing and type of infection. We hypothesised that lower respiratory tract infections (LRTIs) compared with upper respiratory tract infections (URTIs) and early compared with later infections would have a stronger association with adult lung function.

Methods The Barry Caerphilly Growth study collected information on childhood infections (URTI, LRTI and gastrointestinal infections) from birth to 5 years on 14 occasions. Subjects were traced at 25 years of age and had lung function (FEV1, FVC, FEV1/FVC, FEF25-75, and PEFR).

Results 581 subjects had acceptable data for both FEV1 and FVC. Childhood LRTIs (0–5 years) but not URTIs or gastrointestinal infections were negatively associated with all lung function measures except FVC (p<0.05) and showed a dose-response effect. LRTIs in the first year of life and between 2 and 5 years were predictive of PEFR (significant interaction with age at infection p=0.02) but only the former predicted FEV1, FEV1/FVC and FEF25-75 in multivariable models for example, β coefficient for >1 LRTI between 0 and 1 year and FEF25-75 −0.306 (95% CI 0.523 to −0.089, p=0.006) compared with −0.021 (95% CI −0.324 to 0.282, p=0.89) for exposure between 2 and 5 years.

Conclusion LRTIs but not URTIs are associated with an obstructive lung function deficit, especially under 1 year, either due to primary infection-related airways damage or a secondary effect reflecting abnormal airway development. The former explanation, if true, may contribute to socioeconomic differences in obstructive airways disease irrespective of smoking behaviour.