- Background: The role of dairy calcium intake and serum vitamin D concentrations in weight loss is controversial. Objective: The objective was to assess the association of dairy calcium intake and serum vitamin D with weight loss. Design: We analyzed data from participants in the 2-y Dietary Intervention Randomized Controlled Trial (DIRECT) [n = 322; mean body mass index (BMI; in kg/m 2 ): 31; mean age: 52 y]. A representative sample (n = 126) was followed for 6 mo for serum vitamin D changes. Results: Baseline serum 25-hydroxyvitamin D [25(OH)D] concentrations decreased significantly across the tertiles of baseline BMI (25.6 6 8.0, 24.1 6 8.9, and 22.9 6 6.8 ng/mL, respectively; P for trend = 0.02). Baseline concentrations of vitamin D and dairy calcium intake were not associated with subsequent weight loss. However, in repeated-measures models adjusted for age, sex, baseline BMI, total fat intake, and diet group assignment, higher 6-mo tertile levels of dairy calcium intake (median for tertiles: 156.5, 358.0, and 582.9 mg/d, respectively) and serum 25(OH)D (14.5, 21.2, and 30.2 ng/mL, respectively) were associated with increased weight loss across the 2-y intervention (23.3, 23.5, and 25.3 kg, respectively, for dairy calcium; P = 0.043; 23.1, 23.8, and 25.6 kg, respectively, for vitamin D; P = 0.013). In a multivariate logistic regression adjusted simultaneously for age, sex, baseline BMI, total fat intake, diet group, vitamin D concentration, and dairy calcium, an increase of 1 SD in dairy calcium intake increased the likelihood of weight loss of .4.5 kg in the preceding 6 mo [odds ratio (OR): 1.45; P = 0.046]. A similar increase was seen for serum 25(OH)D at the 6-mo point (OR: 1.7; P = 0.009). Conclusion: Our study suggests that both higher dairy calcium intake and increased serum vitamin D are related to greater dietinduced weight loss. This trial was registered at clinicaltrials.gov as NCT00160108. Am J Clin Nutr doi: 10.3945/ajcn.2010.29355.