Rationale. The protective effect of breast-feeding on early respiratory infections is well established, but its relationship to the development of subsequent asthma remains controversial. To clarify these complex issues, we examined the association between lung function and infant feeding practices. Methods. In the Tucson Children's Respiratory Study (CRS), feeding practices were assessed prospectively based on questionnaires completed at enrollment and well child visits. Formula introduction was categorized as having occurred before 2 months (n=143, "early formula introduction", between 2 to <4 months (n=336), or at 4 months (n=200, "longer breastfed (BF)"). Lung function was measured at age 11 and 16. A random effects model was used to assess the relationship of infant feeding practices to measures of lung function. Results. FVC by age 16 was increased by 103±40.0ml, p=0.01, and the FEV1/FVC ratio was lower (-1.9±0.6%, p=0.004) in the longer BF children compared to children with early formula introduction. This effect was modified after stratifying by maternal asthma. Compared to children with early formula introduction, longer BF children with asthmatic mothers had a FVC that was not increased (p=0.7) and an FEV1/FVC ratio (-5.7±2.4%, p=0.02) that was significantly decreased by age 16. Longer BF children with non-atopic, non- asthmatic mothers demonstrated an increased FVC (142±71.1ml,p=0.047) and no decrease in FEV1/FVC (p=0.7) compared to children with early formula introduction. Conclusions. Longer duration of breastfeeding favorably influences lung growth in children. However, in the presence of maternal asthma, longer breastfeeding is associated with decreased airflows.