The VITamin D and OmegA-3 TriaL (VITAL; NCT 01169259) is an ongoing randomized clinical trial in 25,875 US men and women investigating whether taking dietary supplements of vitamin D3 (2000 IU) or omega-3 fatty acids (Omacor (R) 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. The VITAL Kidney Function in Hypertension ancillary study will evaluate the effects of vitamin D or omega-3 fatty acids on kidney function among participants with baseline hypertension.