Effect of breastfeeding on the protective effect on asthma has been studied extensively but remains controversial. Studies regarding the effect of breastfeeding on lung function have also been conflicting. The aim of this study was to determine the influence of breastfeeding on lung function in asthmatic children. We included 555 patients who visited Severance Children's Hospital Allergy Clinic with asthma. Pulmonary function, its bronchodilator response (BDR), fractional nitric oxide, and sputum eosinophils were measured. Parents completed questionnaires with information on feeding practices, family history of allergic disease, exposure to tobacco smoke, and presence of pets. Breastfeeding duration was categorized as not breastfed, breastfed <6 months, and breastfed ≥6 months. Within the asthma group, we stratified by atopic sensitization. We also investigated whether exclusivity of breastfeeding had any modifying effect on lung function. In the asthma group, ratio of forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) significantly increased according to breastfeeding duration: 86.6 ± 8.7 for not breastfed group, 87.2 ± 8.6 for <6 months group, and 88.8 ± 7.7 for ≥6 months group. Within asthma group, only the nonatopic subjects showed a significant increase of FEV1/FVC, maximal midexpiratory flow, and decrease of maximal response to BD according to breastfeeding duration. Increase in FEV1/FVC was seen in the exclusive breastfeeding for ≥6 months group compared with those partially breastfed but FVC was significantly lower in those exclusively breastfed <6 months group compared with those partially breastfed. BDR decreased with breastfeeding duration in the nonatopic asthma group. In conclusion, longer duration of breastfeeding appears to have a favorable effect on lung function in asthmatic children, especially in nonatopic subjects.