Objectives: To examine whether combined vitamin D and calcium supplementation improves insulin sensitivity, insulin secretion, ?-cell function, inflammation and metabolic markers. Design: 6-month randomized, placebo-controlled trial. Participants: Ninety-five adults with serum 25-hydroxyvitamin D [25(OH)D] =55 nmol/L at risk of type 2 diabetes (with prediabetes or an AUSDRISK score =15) were randomized. Analyses included participants who completed the baseline and final visits (treatment n = 35; placebo n = 45). Intervention: Daily calcium carbonate (1,200 mg) and cholecalciferol [2,000-6,000 IU to target 25(OH)D >75 nmol/L] or matching placebos for 6 months. Measurements: Insulin sensitivity (HOMA2 S, Matsuda index), insulin secretion (insulinogenic index, area under the curve (AUC) for C-peptide) and ?-cell function (Matsuda index x AUC for C-peptide) derived from a 75 g 2-h OGTT; anthropometry; blood pressure; lipid profile; hs-CRP; TNF-a; IL-6; adiponectin; total and undercarboxylated osteocalcin. Results: Participants were middle-aged adults (mean age 54 years; 69 Europid) at risk of type 2 diabetes (48 with prediabetes). Compliance was >80 for calcium and vitamin D. Mean serum 25(OH)D concentration increased from 48 to 95 nmol/L in the treatment group (91 achieved >75 nmol/L), but remained unchanged in controls. There were no significant changes in insulin sensitivity, insulin secretion and ?-cell function, or in inflammatory and metabolic markers between or within the groups, before or after adjustment for potential confounders including waist circumference and season of recruitment. In a post hoc analysis restricted to participants with prediabetes, a significant beneficial effect of vitamin D and calcium supplementation on insulin sensitivity (HOMA S and Matsuda) was observed. Conclusions: Daily vitamin D and calcium supplementation for 6 months may not change OGTT-derived measures of insulin sensitivity, insulin secretion and ?-cell function in multi-ethnic adults with low vitamin D status at risk of type 2 diabetes. However, in participants with prediabetes, supplementation with vitamin D and calcium may improve insulin sensitivity.