This study was conducted to determine if vitamin D supplementation is required to prevent rickets in breast-fed infants. Breast-feeding rates are increasing, and there are concerns about whether the vitamin D content of breast milk is sufficient. There are a few treatment trials of vitamin D supplementation in breast-fed infants; these were conducted in northern climates. The authors therefore performed a prospective clinical trial comparing vitamin D supplementation with placebo as control in southern Louisiana. Blood samples and questionnaires were collected at birth, 2, 4, and 6 months of age. There were no cases of rickets observed, and no differences in alkaline phosphatase levels between groups. Thus, there was no evidence that vitamin D supplementation reduced rickets risk in the authors' study population. This suggests that the current recommendations for universal vitamin D supplementation of breast-fed infants throughout the United States may need to be revised.