ABSTRACT This is a renewal application to extend post-intervention observational follow-up for 5 yrs in the VITamin D and OmegA-3 TriaL (VITAL), a randomized, placebo-controlled factorial trial of daily vitamin D3 (2000 IU) and marine omega-3 fatty acids (n-3 FAs; 1-g Omacor® fish-oil capsule, with eicosapentaenoic acid [460 mg] + docosahexaenoic acid [380 mg]) in the primary prevention of cancer and cardiovascular disease (CVD) among 25,871 US men aged ≥50 and women aged ≥55 yrs, including 5,106 African Americans. Median treatment was 5.3 yrs. Funding for the ongoing 2-yr post-intervention follow-up (plus data closeout/analysis) ends on May 31, 2020. A 5-yr extension will yield a median post-intervention follow-up of 7 yrs and median cumulative (intervention + post-intervention) follow-up of 12 yrs. During the trial, although vitamin D did not significantly reduce total invasive cancer incidence in the overall cohort, there was a promising signal for a reduction in total cancer mortality, and subgroup analyses indicated cancer protection in African Americans and those with normal body mass index. Although n-3 FAs did not reduce major CVD events in the total cohort, there were significant reductions in myocardial infarction (MI) and other coronary endpoints; subgroup analyses showed reductions in major CVD events in those with low baseline fish intake and in MI in African Americans. Longer follow-up to account for latency effects and additional research to learn which individuals may be most likely to derive a net supplementation benefit is warranted. Annual surveys will update risk factors and endpoint occurrence. Reported endpoints will be confirmed by medical record review and augmented with Medicare linkage surveillance for maximal and unbiased endpoint capture. Deaths will be ascertained via the National Death Index-Plus. Archived baseline blood/DNA samples will allow examination of whether effects of the trial agents on cancer and CVD vary by (a) genetic factors (targeted variants in genes related to vitamin D metabolism, absorption, or receptor function or n-3 FA synthesis and activation; gene-based ancestry; genetic risk scores); (b) vitamin K and magnesium status; and (c) novel vitamin D and n-3 FA biomarkers. Moreover, continued infrastructure funding will enhance the value of ongoing ancillary studies evaluating the trial agents’ role in prevention of other diseases (23 funded studies to date, including diabetes, cognitive decline, bone disorders, and autoimmune conditions) and allow for future ones, including an examination of intervention effects on tumor molecular markers, DNA methylation, and gene expression. Building upon VITAL’s strengths—including a large, well-characterized, racially diverse cohort with high compliance throughout the trial; archived blood samples; dietary and lifestyle assessments; and rigorously adjudicated endpoints—our proposal offers an exceptionally innovative and cost-efficient (<$100/participant/yr in di...