Objective and Hypotheses. The objective of this study is to evaluate the urine calcium-to-creatinine ratio as a biomarker of vitamin D deficiency. Our hypotheses are 1) that urine calcium-to-creatinine ratio correlates with serum 25-hydroxy vitamin D level in infants and toddlers at risk for vitamin D deficiency and 2) that the urine calcium-to-creatinine ratio can be used to screen children for vitamin D deficiency. These hypotheses will be tested through two specific aims. Specific Aim 1 is to determine the correlation between the urine calcium-to-creatinine ratio with 25-hydroxy vitamin D levels and with biomarkers of rickets in infants and toddlers at increased risk for vitamin D deficiency. Specific Aim 2 is to determine the sensitivity, specificity, and positive and negative predictive values of one random urine calcium-to-creatinine ratio in relation to biochemical evidence of vitamin D deficiency.