Design. This will be an observational study of infants and toddlers at increased risk for vitamin D deficiency. We will recruit 150 healthy children, 6 to 36 months of age, with risk factors for vitamin D deficiency (premature birth, breast feeding, dairy product avoidance, etc.). Based on previous studies in the US, we anticipate about 12% of these subjects will have vitamin D deficiency. Subjects will have a physical exam targeted to signs of rickets. Laboratories will include a random urine sample for calcium-to-creatinine ratio and blood for calcium, creatinine, alkaline phosphatase, phosphorus, magnesium, intact PTH, and 25-hydroxy vitamin D. Subjects with biochemical evidence of rickets will have x-rays done of the left wrist and knees that will be scored according to previously published rickets scores.