This document summarizes a study examining the use of supplemental vitamin D among infants who were breastfed for prolonged periods. The study found that among infants who were predominantly breastfed for at least 6 months, the rate of receiving supplemental vitamin D was only 15.9%. Parental decisions about vitamin D supplementation were significantly associated with whether the parent agreed their pediatrician recommended it and whether they believed breast milk contains all needed nutrition. Educational efforts are needed to increase compliance with guidelines recommending all breastfed infants receive vitamin D supplementation.