Objective: Vitamin D supplementation may be required for certain subgroups in the United States in whom status and intake are inadequate, but the impact of various doses, and whether calcium administration jointly or independently influences vitamin D metabolite levels, is unclear. Methods: In a pilot chemoprevention trial of biomarkers of risk for colorectal adenoma, we measured the impact of vitamin D supplementation and/or calcium supplementation on plasma vitamin D metabolite concentrations. Ninety-two adult men and women living in the southeastern United States were randomized to 800 IU vitamin D3, 2000 mg elemental calcium, both, or placebo daily for 6 months. We examined vitamin D status at baseline and postintervention and compared the change in plasma 25-hydroxyvitamin D (25(OH)D) and 1,25(OH)2D levels by intervention group using general linear models. Results: Eighty-two percent of the study population had insufficient plasma 25(OH)D concentrations (<75 nmol/L) at baseline, with the lowest levels observed among African American participants. Vitamin D supplements, with or without calcium supplementation, raised plasma 25(OH)D concentrations, on average, by 25 to 26 nmol/L. Half of the study participants were classified as having sufficient 25(OH)D status after 6 months of 800 IU of vitamin D3 daily. Calcium alone did not influence 25(OH)D concentrations. Conclusion: In this southeastern U.S. population, half of the study participants receiving 800 IU vitamin D3 daily had blood 25(OH)D concentrations of ≤75 nmol/L after a 6-month intervention period, supporting higher vitamin D dose requirements estimated by some groups. More research is needed to identify the optimal vitamin D dose to improve 25(OH)D status in various at-risk populations.