The Puget Sound Pediatric Research Network (PSPRN) is a practice-based research network of pediatricians in the Puget Sound area and at the University of Washington and Children’s Hospital and Regional Medical Center. The network has extensive experience in conducting large studies on contentious issues that are pertinent to children, their parents, and primary care providers. In 2004, vitamin D supplementation was recommended for all exclusively breastfed infants. This recommendation was based on sporadic reports over the last 30 years of vitamin D deficient rickets in infants and toddlers who have been exclusively breastfed. The recommendation has been controversial because it might undermine efforts to promote breastfeeding, and because the prevalence of rickets in these patients is unknown. PSPRN proposes to conduct a preliminary study to determine the feasibility of a larger planned project by the network to determine the prevalence of rickets in breastfed infants and toddlers who do not receive supplemental vitamin D. Systematic surveys will be conducted in PSPRN practices to determine current levels and trends in vitamin D supplementation. Current practices of PSPRN pediatricians regarding supplementation, and their effect on parents, will also be assessed. The utility of a serum alkaline phosphatase level as a screening test for vitamin D deficient rickets will be determined in the proposed study by obtaining levels on approximately 300 patients, 6-15 months old, seen by PSPRN practitioners, who have been breastfed. In addition to assessing possible methodologies for the larger project, the preliminary study is designed to yield standalone results with important implications.