It has been suggested that breastfeeding may prevent inhibitor formation in patients with hemophilia. In a single-center retrospective cohort study, the authors studied the association between breastfeeding and inhibitor development in 90 severe hemophilia A patients born 1975-2003. Mean follow-up was 13.2 years (standard deviation 8.7). A total of 62% of patients received breastfeeding and 20% developed inhibitors. The inhibitor rate according to breastfeeding was similar: 21% in patients who were breastfed and 18% in others. Patients who were breastfed received their first treatment 0.3 years earlier. After adjustment for age at first treatment, we found no protective effect of breastfeeding on inhibitor formation.