The VITamin D and OmegA-3 Trial (VITAL; NCT 01169259) is a randomized clinical trial in 25,875 U.S. men and women investigating whether taking daily dietary supplements of vitamin D3 (2000 IU) or omega-3 fatty acids (Omacor fish oil, 1 gram) reduces the risk of developing cancer, heart disease, and stroke in people who do not have a prior history of these illnesses. This ancillary study is being conducted among participants in VITAL and will examine whether omega-3 fatty acids or vitamin D3, compared to placebo, reduce the incidence and/or progression of dry eye disease.