Breastfeeding has been linked with increased forced vital capacity (FVC) in children but not in older adolescents. Our aim was to investigate the effects of breastfeeding duration and infant weight gain on FVC in both developmental periods. In a birth cohort, information on breastfeeding duration was collected at 1 and 2 yrs; spirometric tests were conducted at 10 and 18 yrs. To estimate the effect of breastfeeding duration on FVC at 18 yrs of age, we used linear models; to analyse repeated FVC measurements at 10 and 18 yrs of age, we used linear mixed models. Links between breastfeeding, infant weight gain and FVC at 10 and 18 yrs of age were analysed through path analyses. Among 808 breastfed children, 49% were breastfed for ≥ 4 months. At 18 yrs of age the augmenting effect of breastfeeding on FVC was reduced with increased height. Linear mixed models identified that breastfeeding duration was associated with increased FVC. Path analysis suggested a direct effect of breastfeeding on FVC at 10 yrs of age, but an indirect effect at 18 yrs of age via FVC at 10 yrs of age. Although inversely related to breastfeeding, a higher weight gain in infants led to taller adolescents and, in turn, resulted in increased FVC. In conclusion, a longer duration of breastfeeding contributes to lung health in childhood and adolescence.