Objective To examine the associations of duration of exclusive breastfeeding with upper respiratory tract infections (URTI), lower respiratory tract infections (LRTI) and gastrointestinal tract infections (GI) in infancy.

Methods This study was embedded in the Generation R Study, a population-based prospective cohort study from fetal life onwards in The Netherlands. Breastfeeding during the first 6 months (never; partial <4 months, not thereafter; partial 4–6 months; exclusive 4 months, not thereafter; exclusive 4 months, partial thereafter; exclusive 6 months) and the occurrence of doctor-attended URTI, LRTI and GI until the age was 12 months were assessed by questionnaires and available in 4,164 subjects.

Results Prolonged and exclusively breastfed infants had lower risks of URTI, LRTI and GI during the first 6 months, and a lower risk of LRTI between the ages of 7 and 12 months (p-values for trend <0.01). Compared to never breastfed infants, those who were breastfed exclusively until 4 months and partially thereafter, had lower risks of URTI, LRTI and GI until the age of 6 months (adjusted odds ratio (aOR) 0.65 (95% CI 0.51 to 0.83), aOR 0.50 (95% CI 0.32 to 0.79) and aOR 0.41 (95% CI 0.26 to 0.64), respectively) and of LRTI between 7 and 12 months (aOR 0.46 (95% CI 0.31 to 0.69)). Similar tendencies were observed for infants exclusively breastfed ⩾6 months.

Conclusions Prolonged duration of exclusiveness of breastfeeding is associated with lower risks of common infectious diseases in the first 6 months life. These findings support current health policy strategies to promote breastfeeding in industrialized countries.