The objective was to assess associations between duration of total and exclusive breastfeeding and lung function up to adolescence.A birth cohort (Melbourne Atopy Cohort Study) of 620 infants with a family history of allergic disease was recruited. Mothers were encouraged to breastfeed exclusively for 6 months. Lung function was assessed at 12 and 18 years of age. Associations between breastfeeding and lung function were investigated using multivariable linear regression and path analysis was used to assess the potential mediating factors.Duration of breastfeeding (total and exclusive) was not associated with most assessed lung function outcomes. However, there was a trend for increased pre-bronchodilator mid-expiratory flow (MEF) at both 12 (adjusted mean difference (95% CI) per week of breastfeeding of 10 (−1–20) mL·s⁻¹) and 18 years (11 (−1–22) mL·s⁻¹) (p-values of 0.07 and 0.08, respectively). There was a strong indirect effect of height on these observed associations.Duration of breastfeeding does not appear to greatly influence lung function outcomes in children with a family history of allergic diseases. Longer duration of exclusive breastfeeding may be associated with an increase in MEF, partly due to greater attained height of the child.