Background: Epidemiologic data suggest that vitamin D supplementation may reduce cancer mortality. We tested whether vitamin D and/or omega-3 supplementation reduces the incidence of advanced stage cancer at diagnosis or lethal cancer, and whether body mass index (BMI) modifies these associations. Methods: The VITamin D and OmegA-3 TriaL (VITAL) is a randomized, placebo-controlled, 2x2 factorial trial of vitamin D3 (cholecalciferol, 2000 IU/day) and marine omega-3 fatty acids (1 g/day) that enrolled men aged ≥50 years and women aged ≥55 years free of cancer and cardiovascular disease at baseline. For this particular analysis, the primary outcome is a composite of metastatic and fatal invasive total cancer. Secondary analyses included examination of BMI (<25, 25-<30, and >= 30 kg/m²) as effect modifiers of the observed associations. Results: VITAL randomized 25,871 participants, among whom 1,617 were diagnosed with invasive cancer over a median 5.3 year intervention period. No significant differences by treatment arm (vitamin D vs placebo: hazard ratio [HR]=0.96; 95% confidence interval, 0.88-1.06; p=0.47; omega-3 vs placebo; HR 1.03 [0.93-1.13]; p=0.56) were observed. However, a significant reduction in advanced cancers (metastatic or fatal) was found for those randomized to vitamin D, compared to placebo (226 assigned to vitamin D and 274 to placebo; HR 0.83 [0.69-0.99]; p=0.036). There was no difference by omega-3 assignment (246 assigned to omega-3 and 254 to placebo: HR 0.97 [0.81-1.15], p=0.72). When stratified by BMI, there was a significant reduction for the vitamin D arm in incident metastatic or fatal cancer among those with normal BMI (BMI<25: HR 0.62 [0.45-0.86], but not among those who were overweight or obese (BMI 25-<30: HR 0.89 [0.68-1.17]; BMI >=30: HR 1.05 [0.74-1.49]); p for interaction by BMI =0.03. There was no effect modification by BMI noted for the omega 3 arm. Conclusions: In a randomized clinical trial, supplementation with vitamin D, but not omega-3s, reduced incidence of advanced (metastatic or fatal) cancer in the overall cohort, with strongest risk reduction in normal weight individuals. Further research is needed to understand these findings. Clinical trial information: NCT01169259