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To study whether the duration of breastfeeding and time for introduction of complementary foods was associated with the risk of childhood asthma.We used data from the Norwegian Mother and Child Study, a nationwide prospective cohort study that recruited pregnant women from across Norway between 1999 and 2008. Children with complete data of breastfeeding up to 18 months and current age >7 years were eligible (n = 41 020). Asthma as the primary outcome was defined based on ≥2 dispensed asthma medications at age 7 years registered in the Norwegian Prescription Database. We used log-binomial regression models to obtain crude relative risks (RRs) in the main analysis, and adjusted for selected confounders in multivariable analyses.For duration of any breastfeeding, 5.9% of infants breastfed <6 months (adjusted RR [aRR] 1.05, 0.93–1.19) and 4.6% breastfed 6–11 months (aRR 0.96, 0.87–1.07) had dispensed asthma medications at age 7 years compared with 4.6% of infants breastfed ≥12 months (Ptrend .62). Infants still breastfed at 6 months, but introduced to complementary foods <4 months and 4–6 months, had an aRR of 1.15 (0.98–1.36) and 1.09 (0.94–1.27) respectively, compared with infants fully breastfed for 6 months (Ptrend .09). Ages at introduction of solids or formula separately were not significant predictors (Ptrend .16 and .08, respectively).We found no association between duration of breastfeeding or age of introduction to complementary foods and asthma at age 7 years.